If speed is your concern, I would recommend using a pre-built regexp for fast filtering of the candidate words, and a slower exact test against a canonical (sorted letters) form.

Tested, working code:

use strict;
use warnings;

my $dict = '/usr/share/dict/words';

# Do not space-separate the word in this version.
my $scrambled_word   = shift
  or die "Must specify a word\n";

my $scrambled_length = length $scrambled_word;
my $scrambled_sorted = join '', sort split '', $scrambled_word;

my $pattern = qr{
    \A
        (?:
            [$scrambled_word]{$scrambled_length}
        )
        \n
    \z
}x;

open DICT, '<', $dict
  or die "Cannot open '$dict': $!";

while (<DICT>) {
    next unless /$pattern/o;

    chomp;
    my $sorted = join '', sort split '', $_;
    next unless $sorted eq $scrambled_sorted;

    print "$_\n";
}
close DICT or warn;
[download]