my $first;
   ...
  my $second = $first;
  $second =~ s/some/substitution/;

##</code><code>##

my @output = map {
        my $temp = $_;
        $temp =~ s/some/subst/g;
        $temp
    } @input;

##</code><code>##

use Regex::FunctionalSubstitute;
my $first;
 ...
my $second = $first \~ s/some/subst/g;

##</code><code>##

my @output = map {
        $_ \~ s/some/subst/g;
    } @input;

##</code><code>##

package Regex::FunctionalSubstitute;
use Filter::Simple;

use strict;
use warnings;

my $identifier = qr/\$\S+/;
my $operator = qr/\\~/;
my $temp = '$Regex::FunctionalSubstitute::temporary_variable';

=head1 TODO

Test Suite
This documentation
More accurate 'identifier' regex

=head1 Conversion

$this = $that \~ s/one/two/;

converts to:

$this = ( $temp = $that, $temp =~ s/one/two/, $temp )[2]

=cut

FILTER_ONLY(
        code => sub {
                my $ph = $Filter::Simple::placeholder;

                s{
                        ($identifier)  # the source of the substitution
                        \s*            # possible space between
                        $operator      # the operator '\~'
                        \s*            # more possible space between
                        ($ph)          # the regex itself
                }
                { ( $temp = $1, $temp =~ $2, $temp )[2] }gx;

        },
);

1;