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

my $text= "start \\chapter{hello}\n end\n";

my $patternreadfromfile= "-- FOUND \$1 -- ";

my $inpattern = qr/\\chapter\{(.*)\}/;
my $outpattern = q{$patternreadfromfile};

$text =~ s/$inpattern/$outpattern/gee;

print "$text\n";

now q{} escapes too much...