use strict;
use Time::Local;

my ( $dy, $mo, $yr ) = ( localtime )[3..5];

my $ref_time = "08:35:00";  # (pm would be 20:35:00)
my ( $hr, $mi, $sc ) = split( /:/, $ref_time );

my $ref_sec = timelocal( $sc, $mi, $hr, $dy, $mo, $yr );

my $delta_time = "01:23:00";
my ( $add_hr, $add_mi, $add_sc ) = split( /:/, $delta_time );
my $delta_sec = (($add_hr * 60) + $add_mi) * 60 + $add_sc;  

( $sc, $mi, $hr, $dy, $mo, $yr ) = localtime( $ref_sec + $delta_sec );

# don't forget to add 1 to $mo, and 1900 to $yr if/when you use them;

# one way to check results:

print scalar( localtime( $ref_sec + $delta_sec )), $/;