There are a whole bunch of logging modules on CPAN. Log::Dynamic looks suitable to my undiscerning eye.

Update: You can get the effect you want — having $now implicitly call localtime — by tie-ing it:

package Now;
require Tie::Scalar;

our @ISA = qw(Tie::StdScalar);

sub FETCH {scalar localtime}

package main;

tie my $now, 'Now';

print "The time is $now\n";
sleep 3;
print "The time is $now\n";
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It's probably not what you really want to do, though.