Another option is to use a subroutine reference instead of a string:

#! perl
use strict;
use warnings;

my @arr = (1, 2, 3, 4, 5);
my $pat = sub { $_[0] < 3 };
my @res;

@res = grep {$_ < 3} @arr;
print "Literal:\t@res\n";

@res = grep { $pat->($_) } @arr;
print "Variable:\t@res\n";
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Output:

 0:52 >perl 939_SoPW.pl
Literal:        1 2
Variable:       1 2

 0:52 >
[download]

Hope that helps,