#!/usr/bin/perl
use strict;
use warnings;

my @lines = split /\n/, <<'END';
begin{verbatim}
hello all
end{verbatim}
END

s{((?:begin|end))\{verbatim\}}{$1\{whatever\}} for @lines;
print "$_\n" for @lines;

__END__
poletti@Polebian:~/sviluppo/perl/pwc/tmp$ perl bug.pl
Use of uninitialized value in substitution iterator at bug.pl line 11.
Use of uninitialized value in substitution iterator at bug.pl line 11.

hello all