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

my $found_f = 0;
my ($lastline, $line);

open (INPUTFILE, "test.txt") or 
    die "Unable to open test.txt: $!";   # 'or die' Added

while (<INPUTFILE>)
{
    chomp;

    if (/^[A-Z]{2}:\s/) {
        $lastline = $line;
        $line = $_;
        $found_f = 1;
    }
    elsif ($found_f) {
        s/^ {4}/ /;
        $line .= $_;
    }
    elsif (/^$/) {
        $lastline = ' ';
        $found_f =0;
    }

}                      # Added

$lastline .= $line;    # Added - this was your main problem

print "$lastline\n";
close (INPUTFILE);     # Added