gistfile1.txt

use Moose::Util::TypeConstraints;
use MooseX::Params::Validate;

subtype 'Filename' => as 'Str' => where { -x $_ };

sub foo {
    my ($name) = pos_validated_list( \@_, { isa => 'Filename' } );
    print "Houston, we have a filename: $name \n";
}

my $perl = `which perl`; chomp $perl;

foo($perl);
foo('no_such_file');

gistfile2.txt

stevan% perl ovid.pl
Houston, we have a filename: /usr/local/perls/bin/perl
Parameter #1 ("no_such_file") to main::foo did not pass
  the 'checking type constraint for Filename' callback ...