One way is to put the specific code that is generating the warning into a block that redirects STDOUT. You'll want to ensure you limit the scope, because if any other warnings are generated, you won't see them.

#!/usr/bin/perl
use warnings;
use strict;

sub warning {
    print "ERROR!!!\n";
}

my $out;
open my $stdout, '>', \$out
  or die $!;

{
    local *STDOUT = $stdout;
    warning();
}
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-stevieb