An example of monkey patch

monkey.pl

use strict;
use warnings;
use Time::Piece;

{
    no warnings 'redefine';
    my $orig_strptime = \&Time::Piece::strptime;
    *Time::Piece::strptime = sub {
        my @warns;
        my $result = do {
            local $SIG{__WARN__} = sub { push @warns, @_ };
            $orig_strptime->(@_);
        };

        warn "CAPTURED: $_" for @warns;
        warn "caller: " . caller if @warns;

        $result;
    };
}

Time::Piece->strptime('2015-12-27 13:19:00', '%Y-%m-%d');