You could use regexps, like this:

$needle = "a";
$haystack =~ /\Q$needle\E/i;
$index = $-[0];
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this works even if $needle has regexp-metacharacters in it.

But @- exists only in newer perls (from 5.6.1 I think). In perl 5.005 you could use something like

$needle = "a";
$haystack =~ /\Q$needle\E/i;
$index = pos($haystack)-length($needle);
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or

$needle = "a";
$haystack =~ /\Q$needle\E/i;
$index = length($`);
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but note that $` makes perl slower.

Whitespace added by tye