Often, I put parameter and other validation routines into 'private' subs (leading underscore on a sub name is not enforced, but it's widely known not to use these special functions directly). They are often fit for a single purpose, and not reused. Here's an extremely trivial example. Now, if there's a bug found where we need another check, it goes into the validation function, and doesn't pollute the main subroutine:

use warnings;
use strict;

sub func {
    my ($x, $y) = @_;

    _validate($x, $y);
    print "$x, $y\n";
}
sub _validate {
    my ($x, $y) = @_;

    die if $x <= 20;
    die if $y !~ /^\w\d{2}/;
    die if length $y > 10;
    die if $y =~ /y/;
}
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This is especially handy when using an editor that folds your subs for you...