strange error in line 7

gistfile1.pl

#!/usr/bin/perl
use strict;

sub test
{
    open FP, "< $0" or die;
    while (<FP>)    # Modification of a read-only value attempted at ./test.pl line 7.
    {
        chomp;
        print "w: $_\n";
    }
    close FP;
}

for (qw(a b))
{
    print "f: $_\n";
    test();
}