sub match { no warnings; my $value = shift @_; $value =~ s{^(?:aa(bb))?.*}{$1}i; return $value; } sub match_warn { use warnings; my $value = shift @_; $value =~ s{^(?:aa(bb))?.*}{$1}i; return $value; } print match('aabbcc')."\n"; # 'bb' print match('abbcc')."\n"; # '' print match_warn('aabbcc')."\n"; # 'bb' print match_warn('abbcc')."\n"; # Use of uninitialized value $1 in substitution iterator