use strict;
use warnings;

use Test::More tests => 2;
use Test::Warn;

# Your code which gives an expected warning
sub foo {
    warn "Stripe Webhook Error: Invalid Stripe Signature\n";
}

sub bar {
    STDERR->print ("Stripe Webhook Error: Invalid Stripe Signature\n");
}

# Test your code
warning_is { foo (); } "Stripe Webhook Error: Invalid Stripe Signature\n",
    'Invalid sig warning issued';
warning_is { bar (); } "Stripe Webhook Error: Invalid Stripe Signature\n",
    'Invalid sig STDERR print issued';