#!/usr/bin/perl -i.bak
  :

##</code><code>##

$ ./myfilter -i file_a file_b

##</code><code>##

while (<filea>) {
    if ( this is a line i want ) {
        s/[\015\012]+$//; # Just to remove any CR/LF
        my($router,$site)= split /,/,$_,3;
        $replace{$router}= $site;
    }
}
my $replacements= '\b('.join('|',keys %router).')\b';

##</code><code>##

my $symbol;
while (<>) {
    if (my $hit/^SYMBOL: $replacements/i ... /^END SYMBOL/i) {
        if ($hit==1) {
            $symbol=$1;
        }
        else {
            s/^(Parent Submap:).*/$1 $replace{$symbol}/;
        }
    }
    print;
}