Well, what have you tried?

There is, of course, more than one way to do this. One way is to use grep:

my @after;
@after = grep { !/[GT]/ } @positions;
[download]

which is, in essence,

my @after;
foreach @positions {
    push(@after, $_) if (!/[GT]/);
}
[download]

I tend to use parenthesis where perl's syntax doesn't require them, but that's just me. I find they frequently improve readability.