use strict;
my $str = '# match String Cool! only if there is a : somewhere preciding it
Ein String: mit einem Zeichen drin Cool!
Ein String ohne Zeichen drin Cool!';
my $regex = qr/(:.+)(?(1)Cool!)/mp;
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