Vim

Objectively, vim is the best editor to use for writing code in, except when it isn't. Here are some things to make it better: 
=== Perltidy ===
This will make pressing Ctrl+t tidy the whole file. Alternately, you can use visual mode to select a block to tidy. 
" Allow easy running of perltidy when editing a perl file, bound to 'Ctrl+t' 
:au Filetype perl nnoremap <C-t> :%!perltidy -q<CR> 
:au Filetype perl vnoremap <C-t> <line1>,<line2>!perltidy -q<CR>
With this, pressing Ctrl+t will tidy the whole file.
However, using V to select a block, pressing Ctrl+t will only tidy that block.
This is good for cleaning up the code around where you've been working if it's not adhering to a style. 
Some info on Koha's offical perltidy style is here
=== Perl-support ===
Perl-support adds many Perl helper functions to your environment, giving you quick access to all sorts of useful things. There's also a handy printable key reference list there.
=== Perl-debugging ===
See Debugging in VIM for a guide to setting up interactive debbuging from within vim.
=== .vimrc ===
Here are a number of nice additions one can put in .vimrc 
if empty(glob('~/.vim/autoload/plug.vim'))
 silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs
 \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
 autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif
" Specify a directory for plugins
" - For Neovim: stdpath('data') . '/plugged'
" - Avoid using standard Vim directory names like 'plugin'
call plug#begin('~/.vim/plugged')
Plug 'vim-perl/vim-perl'
Plug 'scrooloose/syntastic'
Plug 'bling/vim-airline'
Plug 'tpope/vim-fugitive'
Plug 'prettier/vim-prettier', { 'do': 'yarn install' }
" Initialize plugin system
call plug#end()
"set syntax highlighting on by default
syntax on
"Add line numbers along the left-hand side of the screen
set number
" size of a hard tabstop
set tabstop=4
" size of an "indent"
set shiftwidth=4
" a combination of spaces and tabs are used to simulate tab stops at a width
" other than the (hard)tabstop
set softtabstop=4
" make "tab" insert indents instead of tabs at the beginning of a line
set smarttab
" always uses spaces instead of tab characters
set expandtab
" always show filename at the bottom of the screen
set modeline
set ls=2
"define :Tidy command to run perltidy on visual selection || entire buffer"
command -range=% -nargs=* Tidy <line1>,<line2>!perltidy
"run :Tidy on entire buffer and return cursor to (approximate) original position"
fun DoTidy()
 let Pos = line2byte( line( "." ) )
 :Tidy
 exe "goto " . Pos
endfun
"shortcut for normal mode to run on entire buffer then return to current line"
au Filetype perl nmap <F6> :call DoTidy()<CR>
"shortcut for visual mode to run on the the current visual selection"
au Filetype perl vmap <F5> :Tidy<CR>
" Enable HTML syntax highlighting for Template Toolkit files
au BufRead,BufNewFile *.tt set filetype=html
" Enable HTML matching tag jumping with shift-% just like matching bracket jumping
runtime macros/matchit.vim
"Assuming autoindent and smartindent are set correctly, typing Ctrl + Return between braces will put your cursor where you want it to be.
set autoindent
set cindent
" Uncomment the following to have Vim jump to the last position when reopening a file
if has("autocmd")
 au BufReadPost * if line("'\"") > 1 && line("'\"") <= line("$") | exe "normal! g'\"" | endif
endif