#!/usr/bin/perl                                                                                                 
                                                                                                                  
use strict;
use warnings;

my ( @l, @u, @n, @p, $i, $j );

open( URANDOM, "</dev/urandom" ) || die $!;
read( URANDOM, $_, 4 );
close URANDOM;

srand( unpack( "L", $_ ) );

@l = ( "a" .. "z" );
@u = ( "A" .. "Z" );
@n = ( 0 .. 9 );

push( @p, splice( @l, int( rand(@l) ), 1 ) ) for ( 1 .. 10 );
push( @p, splice( @u, int( rand(@u) ), 1 ) ) for ( 1 .. 10 );
push( @p, splice( @n, int( rand(@n) ), 1 ) ) for ( 1 .. 2 );


for ( $i = @p ; --$i ; ) {
    $j = int( rand( $i + 1 ) );
    next if $i == $j;
    @p[ $i, $j ] = @p[ $j, $i ];
}

print join( "", @p ) . qq(\n);

__END__

Karls-Mac-mini:monks karl$ ./random_filename.pl 
gRjyL1ivolIPHsJx0MCSeQ