Though the other answers are probably more desirable, just for comparison, here's what you actually asked for (untested):
# You may also want additional substitutions
# to backwhack  sed metacharacters, e.g.
# $search =~ s/\./\\./g;
$search =~ s/([[:alpha:]])/"[".lc($1).uc($1)."]"/ge;
system "sed -e s/$search/$replace/g $write1 > $write2";

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But it's much easier to use the case insensitive regex switch in perl :-)
Update: Updated code. In reply to AM: I'm just creating sed character classes, which, e.g., replaces "a" with "[aA]" to make it case insensitive.

In reply to Re: how to make sed case INsensitive by runrig
in thread how to make sed case INsensitive by Anonymous Monk

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