Have you read Reporting Warnings from a Module? This can be used to achieve precisely what you have outlined as your goal here. eg:

package Foo;

use warnings::register;

sub bar {
    $^W = warnings::enabled () ? 1 : 0;
    print $baz;
};

1;

package main;

use warnings 'Foo';
Foo::bar ();

no warnings 'Foo';
Foo::bar ();
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If you run this you will only see one occurrence of the warning Use of uninitialized value $Foo::baz in print at /tmp/warnshow.pl line 9. which is caused by the first call to Foo::bar (). The second call does not generate the warning. HTH.