use warnings;
use strict;

$SIG{__WARN__} = sub {
    my $warn = shift;
    if ($warn !~ /^A thread exited/){
        print $warn;
    }
};

warn "blah blah warning\n";
warn "A thread exited...\n";

##</code><code>##

blah blah warning

##</code><code>##

perl -E '$SIG{__WARN__}=sub{say "handler";warn $_[0] if $_[0]!~/^A thread/}; warn "blah"; warn "A thread";'
handler
blah at -e line 1.
handler