[petre]

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).

[chromatic]

You're right, that's exactly what I meant!

I'm surprised there's no example in perlre(1); perhaps I can get that corrected.