use strict;
my $str = 'blue cheese, a red nose.
bleu rouge';
my $regex = qr/(?sx)
(?| # branch reset: both captures go to Group 1
\bblue\b(?=.*(bleu))
|
\bred\b(?=.*(rouge))
)/sxp;
my $subst = '\\1';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html