As a general rule, you should avoid changing an array you are iterating over. That's bound to cause all kinds of headaches. Also, it is favourable to name variables by what they mean, not by what they contain. Here, $num is a bad name. $index is better, so are $i, $j, $k, etc.

Here's my stab at the problem.

use strict;
use warnings;

my @colors = qw(red green blue yellow pink purple brown);
my @drop = qw(pink brown);


foreach my $drop_color (@drop)
{  
    my $index = 0;
    foreach my $color (@colors)
    {
        if ( $color eq $drop_color )
        {        
            splice (@colors, $index, 1);
            last;
        }
        $index++;
    }
}
print "@colors \n";
[download]

Besides being clearer, this also works with n elements in whatever array, whereas your solution hardcodes two elements in the @drop.

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