simplify the conditionals

original code (stripped for clarity)

 1  my $stdout_line = "";
 2
 3  my @stdout      = ();
 4  @stdout = <$stdout_fh>;
 5
 6  my $stdout_line = $stdout[0]
 7    if (scalar (@stdout));
 8
 9  foreach my $err (@error_codes) {
10    my $rx_stdout  = $err->{'stdout_regex'};
11
12    return $err->{'error'}
13      if ( ($rx_stdout  eq q{} || $stdout_line =~ /$rx_stdout/ ) ...

The term '$regexp eq q{}' in.

  if ($regexp eq q{} || $value =~ /$regexp/) ...

is superfluous, because '$value =~ //' will always be true anyway.

 1  my $stdout_line = "";
 2
 3  my @stdout      = ();
 4  @stdout = <$stdout_fh>;
 5
 6  my $stdout_line = $stdout[0]
 7    if (scalar (@stdout));
 8
 9  foreach my $err (@error_codes) {
10    my $rx_stdout  = $err->{'stdout_regex'};
11
12    return $err->{'error'}
13      if ( $stdout_line =~ /$rx_stdout/  ...

The initialization in line 1 can be replaced with a test whether '@stdout'
contains at least one line.

 1  my @stdout      = ();
 2  @stdout = <$stdout_fh>;
 3
 4  my $stdout_line = $stdout[0]
 5    if (scalar (@stdout));
 6
 7  foreach my $err (@error_codes) {
 8    my $rx_stdout  = $err->{'stdout_regex'};
 9
10    return $err->{'error'}
11      if (@stdout and $stdout_line =~ /$rx_stdout/  ...

This makes it possible to get rid of '$stdout_line'.

 1  my @stdout      = ();
 2  @stdout = <$stdout_fh>;
 3
 4  foreach my $err (@error_codes) {
 5    my $rx_stdout  = $err->{'stdout_regex'};
 6
 7    return $err->{'error'}
 8      if (@stdout and $stdout[0] =~ /$rx_stdout/  ...

There is no need to initialize @stdout since it will be unconditionally
replaced with the content of $stdout_fh.

 1  my @stdout = <$stdout_fh>;
 2
 3  foreach my $err (@error_codes) {
 4    my $rx_stdout  = $err->{'stdout_regex'};
 5