It's easier when you read the directions we pointed to: hex, chr.

use warnings;
use strict;
use Test::More;

my $bar = "\x04\x4d";
my $foo = "044d";
my $string;

while ($foo =~ m/(..)/g) {
        $string .= chr(hex($1));
}

is( $bar, $string, '$bar eq $string' );
done_testing();

print "string: $string\n";
print "bar: $bar\n";

__END__

ok 1 - $bar eq $string
1..1
string: M
bar: M
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Update: For the record, Test::More, is(), and done_testing() are not significant to the solution. I used them to demonstrate the equality.