The winner seems to be File::Map. It is less complicated to use, and at least performs in correct accordance to its documentation (unlike Sys::Mmap). ++ to leont once again.

use strict;
use warnings;

use File::Map 'map_file';

map_file my $map, '/usr/share/dict/words', '<';
while( $map =~ m/^(wal.*)$/mg ) {
  print "$1\n";
}
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Though its user interface is fairly simple, it is a module where reading 100% of the documentation is probably a very good idea.