c:\@Work\Perl\monks>perl -MData::Dump -le
"use strict;
 ;;
 my $rx = 'x';
 my @ra = grep !/$rx/, undef;
 dd \@ra;
"
[undef]

c:\@Work\Perl\monks>perl -MData::Dump -le
"use strict;
 use warnings;
 ;;
 my $rx = 'x';
 my @ra = grep !/$rx/, undef;
 dd \@ra;
"
Use of uninitialized value $_ in pattern match (m//) at -e line 1.
[undef]