One way is to copy one hash and convert all keys to lower case. This may be costly if the hash is big:

use strict;
use warnings;

my %hash1 = ("TEXT", 25);
my %hash2 = ("text", 25);
my %hash1a;

for (keys %hash1) { $hash1a{lc $_} = $hash1{$_} }

for (keys %hash2) {
    if (exists $hash1a{lc $_}) { print "matches\n";}
}
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prints:

matches
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Update: This will work even if the keys are mixed case: teXT. If the keys are all upper case or all lower case, then FunkyMonk's solution is simpler