I can suppress all warnings using $^W

No, warnings overrides $^W/-w (regardless of their relative order). 

use warnings;
BEGIN { $^W = 0; }
my $s = '' . undef();  # Use of uninitialized value

[download]

Not just use warnings;. no warnings; overrides $^W/-w too (regardless of their relative order). 

no warnings;
BEGIN { $^W = 1; }
my $s = '' . undef();  # [No warnings]

[download]

In the context of your problem, $^W won't work if the sub was compiled with use warnings; in effect. 

use strict;
use warnings;

sub test (&) {
    my $code = shift;
    local $^W = 0;
    $code->( undef );
}

test {
    print "@_";   # Use of uninitialized value
};

[download]

That leaves you with $SIG{__WARN__}. You can catch the warnings and filter out the ones you don't want.

Update: Adjusted formatting for coherence. Added example to show that no warnings; overrides $^W too.

In reply to Re: Imposing no warnings xxx upon callback code? by ikegami
in thread Imposing no warnings xxx upon callback code? by BrowserUk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.