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

[code to open database]

sub decode {
    my $encoded_data = shift;
    [code to decode $encoded_data to $decoded_data]
    return $decoded_data;
}

my $blob;
[code to get column from database and put into $blob]
my $pdf = &decode($blob);
open(OUT_PDF, ">DOCUMENT.PDF") or die "Can't open file!";
print OUT_PDF $pdf;
close(OUT_PDF);