JavaFan is right, as that is exactly what the __WARN__ and __DIE__ handlers are intended for:

use strict;
use warnings;

# Customize Warning messages.
BEGIN {
    $SIG{'__WARN__'} = sub { warn localtime . ': ' . $_[0] };
    $SIG{'__DIE__'} = sub { die localtime . ': ' . $_[0] };
}

my $foo = 10;
my $foo = 20; # warning about duplicate my $foo,

die "Goodbye world";
[download]