A quick observation:

while (my $key1 = shift @keys) {
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is a potential bug. Consider what happens when @keys contains an element that Perl considers “false”:

 3:19 >perl -wE "my @keys = ('a'..'c', '0', 'd'..'f'); say ''; while (
+my $key = shift @keys) { print qq[$key: ]; for (@keys) { print qq[$_ 
+] } say '' }"

a: b c 0 d e f
b: c 0 d e f
c: 0 d e f

 3:19 >
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Better to play it safe by testing the size of the array before using shift:

 3:19 >perl -wE "my @keys = ('a'..'c', '0', 'd'..'f'); say ''; while (
+@keys) { my $key = shift @keys; print qq[$key: ]; for (@keys) { print
+ qq[$_ ] } say '' }"

a: b c 0 d e f
b: c 0 d e f
c: 0 d e f
0: d e f
d: e f
e: f
f:

 3:19 >
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madM, I know this doesn’t address your question, but it may just save you some debugging down the track.