Try index. This assumes $[ is 0 as it ought to be.

 
$str1 = $str2 if index( lc($str2), lc($str1)) != -1;
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You don't exactly say you want case insensitivity, but example 2 comment seems to imply you do, so I threw it in. Remove the lc() functions if you don't want it

After Compline,
Zaxo