Nothing is modified, not even letters. Case is preserved. 
>perl -le "'aA' =~ /(.*)/i && print $1
aA

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Only the matching is affected. /i causes lower- and upper-case letters to match lower- and upper-case versions of itself.

Maybe you were thinking of 

>perl -le "lc('aA') =~ /(.*)/ && print $1
aa

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In reply to Re^4: Making a regex case insensitive by ikegami
in thread Making a regex case insensitive by Win

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