I am not sure that your data looks like, but consider this:

while (<FILE>)
{
   chomp;
   my ($first_thing, @cmds) = split(/:/,$_);
   
   print "$first_thing\n";
   print "Individual Elements\n\n";
   foreach my $cmd (@cmds)
   {
      print "  $cmd\n";
   }
}
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Update: don't "slurp" a file into a @var unless you need to do that.
process line by line instead.
I think Corion already pointed out that split works on a scalar, not a @var