You can use an appropriate slice, like this:

my @array = qw/the that which who when if this/;

foreach( @array[2 .. $#array] ) {
        $_ = uc $_; # $_ is an _alias_
}

print "@array\n";
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Unfortunately you have to use $#array (I'd like if I could write things like @array[2:] but after all this is syntactic sugar). Note that you can't write 2 .. -1 to mean from the element with index 2 to the last one.