#!/usr/bin/perl

my $binfile = 'binfile';
my $outfile = 'testfile';

my $re1 = qr{blorf};    # that is the regex which saves the token
my $re2 = qr{foo bar};  # that is the regex for following lines

open my $in,  '<', $binfile or die "'$binfile': $!\n";
open my $out, '>', $outfile or die "'$outfile': $!\n";

my $buf;    # empty
my $found;  # not yet

while(! eof $in) {
    if ($found && /$re2/g) {
        # do substitution
        s/$re2/quux $1 $found/;
    } elsif (/($re1)/g) {
        # if we match the start of a section,
        # we remember what we need and write all things read
        # up to the position of the current match.
        $found = $1;
        my $chunk = substr $_,0,length($1)+pos;
        print $out $chunk;
        substr $_, 0, length($1)+pos,'';
    } else {
        read($in, $buf, 50);
        $_ .= $buf;
    }
}
print $out $_; # spit out last chunk