Here is a good one, which frodo72, now known as polettix showed me in perl encryptions keys vs. c. I was trying to find out how to make c's RC4 compatible with perl's RC4. Anyways, this code is pretty good for your purposes, it uses no modules, and uses a key.

#!/usr/bin/perl -0777  --

# by frodo72 of perlmonks
# $0 key infile > outfile
# symmetric works both ways

use strict;
use warnings;

my @k = map { ord($_) } split //, shift;

my ( @s, $x, $y );
$y = 0;
for ( my @t = @s = 0 .. 255 ) {
    $y = ( $k[ $_ % @k ] + $s[ $x = $_ ] + $y ) % 256;
    &S;
}

$x = $y = 0;

for ( unpack( 'C*', <> ) ) {
    $x++;
    $y = ( $s[ $x %= 256 ] + $y ) % 256;
    &S;
    print pack( 'C', $_ ^= $s[ ( $s[$x] + $s[$y] ) % 256 ] );
}

sub S { @s[ $x, $y ] = @s[ $y, $x ] }
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