my $data = <<'END';
foo123bar
# foo123bar foo456bar
# xyz foo789bar abc
foo456bar
END

$data =~ s{
	^                 # beginning of line
	\h* \# \h*        # comment lines
	\K                # keep everything up to here in replacement
	(?<comment> \N*)  # capture the comment
	$                 # end of line
} { handle_foobar( $+{comment} ) }msxge;

sub handle_foobar {
	return shift =~ s{
			foo \K (\d+) (?= bar )
		}{
			$1 =~ tr/0-9/a-j/r
		}msxger;
}

##</code><code>##

my $str = <<'END';
Hello
# START
World
# END
Aaa
# START
Bbb
# END
Ccc
END

open my $fh, '<', \$str or die $!;
while (<$fh>) {
	chomp;
	if ( /^# START/ .. /^# END/ ) {
		print "<$_>\n";
	}
}
close $fh;