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

my $found_f = 0;
my ( $lastline, $line );
open( INPUTFILE, "test.txt" );

while (<INPUTFILE>) {
    chomp;

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