Does the following come close to what you are looking for? I've modified your snippet so that a pattern is actually read from a file. Putting the s/// operation inside a string eval is an effective way to do what you want (my version does have "FOUND hello" in its output ). Note the backslash before "$text", so that this variable does not get interpolated into the string before it is evaluated as code:

#!/usr/bin/perl

use strict;
use warnings;

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

my $patternreadfromfile = <DATA>;
chomp $patternreadfromfile;

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

eval "\$text =~ s/$inpattern/$patternreadfromfile/g";

print "$text\n";

__DATA__
-- FOUND $1 --
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(update: sorry, this node should have been a reply to your other node in this thread, at Re^2: simple regular expression)