# made-up example with actual issue, with strict and warnings $ perl -Mstrict -Mwarnings -le 'my $str = q{zxcv}; if ( $str -~ m/some pattern/i ) { print $str; }' Use of uninitialized value $_ in pattern match (m//) at -e line 1. Argument "zxcv" isn't numeric in subtraction (-) at -e line 1. zxcv