Don't forget about such tools as YAPE::Regex::Explain, where the synopsis in the POD provides this simple example:

use YAPE::Regex::Explain;
my $exp = YAPE::Regex::Explain->new($REx)->explain;
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In the above example, let $REx be your regular expression as a single-quoted string or a qr// object.

If you print $exp, you'll get a nice table explaining each subexpression within the regular expression.

That's probably one of the more powerful tools for me.

Here is an example output from YAPE::Regex::Explain, when invoked as follows:

perl -MYAPE::Regexp::Explain -e "print YAPE::Regex::Explain->new('\bte
+st(?: more)\b$')->explain();"
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And the output...

The regular expression:

(?-imsx:\btest(?: more)\b$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  test                     'test'
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
     more                    ' more'
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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Perl also provides the use re qw/debug/; pragma, which can be used like this:

#!/usr/bin/perl

use re 'debug';

m/
  \b
  test regexp
  \b
/x;
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When you execute that you'll get a sort of compilation version of the regular expression, which is sometimes helpful in seeing what's going on (or wrong). This is discussed in perldebug and perldebuguts.