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#!/usr/bin/perl
use strict;
...
    while @first || @second;

print "@interleave";

#!/usr/bin/perl
use strict;
...
splice @first, @second, 0, pop @second while @second;

print "@first";

...
splice @first, $_, 0, $second[$_ - 1] for reverse 1 .. @second;

print "@first";

my @new;

splice @new, 0, 0, $first[$_], $second[$_] for reverse 0 .. $#second;
print "@new";

#!/usr/bin/perl
use strict;
...

    printf "%-10s %s\n", $msg, join '|', map {"@$_"} @arrays;
}

start      Can unlock secret|you the code?
first      Can you the unlock secret|you the code?
...
na1        Can you the unlock secret|you the code?|Can you unlock secr
+et you code?
na2        Can you the unlock secret|you the code?|Can you unlock secr
+et code?
na3        Can you the unlock secret|you the code?|Can you unlock the 
+the secret code?