Needs more cowbell /es. And slightly different quoting.

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

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

my $inpattern = qr/\\chapter\{(.*)\}/;
my $outpattern = q{" -- FOUND $1 -- "};

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

print "$text\n";
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