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@oldstock = ("widgets:10:5",
             "wodgets:4:2",
...
@newstock = ("screwdrivers:8:3",
             "hammers:5:1",
             "widgets:3:1");

@totalstock = ("widgets:13:6",  # Both widget entries joined
               "wodgets:4:2",
               "spanners:5:3",
               "screwdrivers:8:3",
               "hammers:5:1");

my %stock = ();
my %shown = ();
...
foreach my $name (keys %stock) {
    print join(":",$name,$stock{name},$shown{name}),"\n";
}