Hi,
quite simple indeed:

my $db_write = shift || 1;
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means: give me the parameter and if it is false (and '0' is false), then set it to 1.
You might try something like this (using defined instead of just checking for true/false):

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

#=====================================================================
sub handleVoicemailNote {
#=====================================================================
    my ($ref_mail,$db_write) = @_;
    $db_write = 1 unless defined($db_write);
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Regards,
svenXY