while (my $key1 = shift @keys) {

##</code><code>##

 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 >

##</code><code>##

 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 >