Is it possible to use a variable in place of the expression or block of grep? Am I missing something simple or is this simply illegal?

With the simple fragment below, I would like to get a result of 1,2 for both greps but I get all elements on the second. Btw, I get the same results from the expression version: "grep $var, @arr".

use strict; use warnings;

my @arr = (1, 2, 3, 4, 5);
my $pat = '$_ < 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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Of course, this is just a test script. The real thing builds the grep pattern in several steps and searches a two-dimensional array. I could do all this differently but the most obvious alternative would be a long list of if statements.