#!/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__
begin{whatever}
hello all
end{whatever}