#!/usr/bin/perl
use strict;
use warnings;
my $prefix;
while (<DATA>){
    chomp;
    next unless $_;
    if ($_ =~ /^!/){
        $prefix = $_;
        next;
    }
    else{
        print "$prefix.$_\n";
    }
}
__DATA__
!Denver

Line 1
Line 2
Line 3

!New York

Line 1 
Line 2