This would seem to be a good chance to use exceptions.

Stylistically, I'd probably use sub refs instead of the string eval, too. If your pre- and post- conditions are in a package, UNIVERSAL::can comes to the rescue. Otherwise, this is an opportunity for a little symbol table muckery:

sub validate {
    my $input = shift;
    my %arg = @_;

    for (@{$arg{as}}) {
        my $func = "validate_$_";
        my $package = __PACKAGE__ . '::';
        no strict 'refs';

        if (exists ${ $package }{$func}) {
            $func = *{ ${ $package }{$func} }{CODE};
            return unless $func->($input);


        } else {
            # throw exception and/or return
        }
    }

    warn "validated $input";
    return 1;
}
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Or you could build a hashref of validators, or cache them instead of explicitly going to the symbol table, or use the Listener pattern, or do all sorts of things.