Um, I think this suggestion, as you originally posted it, will not work as you intend/expect:

@array1 = @array1[+grep 
                   { $array1[$_] =~ /keep these/ }
                   0 .. $#array1
                 ]; 
@array2 = @array2[+grep
                   { $array1[$_] =~ /keep these/ }
                   0 .. $#array1
                 ];
[download]

When you process @array2, @array1 is presumably shorter than it used to be -- the elements of the two arrays are no longer in parallel (assuming they were parallel before the first grep operation). You'll be getting the wrong elements left behind in @array2 (only the last elements of @array2 will ever be removed).