Doesn't work. You don't assign anything to $b, so it's undef: % perl -MData::Dumper=Dumper -e 'my $a = 'Perl'; my $b =~ s/pe/ea/ir; print Dumper $b;' $VAR1

Doesn't work. You don't assign anything to $b, so it's undef: % perl -MData::Dumper=Dumper -e 'my $a = 'Perl'; my $b =~ s/pe/ea/ir; print Dumper $b;' $VAR1 = undef; You probably mean: my $a = 'Perl'; my $b = $a =~ s/pe/ea/ir; Anyway, thank you for the /r modifier, it didn't know what it did, since there's no example in perlre(1).