use strict;
use warnings;
use Sys::Mmap;

my $string;

open my $fh, '<', '/usr/share/dict/words' or die $!;

Sys::Mmap::mmap( $string, 0, MAP_SHARED, PROT_READ, $fh );

while( $string =~ m/^(wal.*)$/mg ) {
  print $1, "\n";
}

Sys::Mmap::munmap( $string ) or die "munmap: $!";
close $fh;

##</code><code>##

variable is not a string at ./mytest.pl line 17.

##</code><code>##

if(SvTYPE(var) != SVt_PV) {
            croak("variable is not a string");
            return;
        }