Where does the with keyword come from and what do you think it does?

If you want a really verbatim translation, you need to replace the With X from VB with for (X) { in Perl:

for ($app->Selection) {
    $_->{ColorIndex} = 8;
};
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I recommend that you instead move away from the (application) global variables like $app->Selection and instead use Perl variables:

my $sel = $app->Range("A1");
my $interior = $sel->Interior;
$interior->{ColorIndex} = 8;
$interior->{Pattern} = xlSolid;
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If you want to translate from VB to Perl, you need to know about both languages. Wild throwing around of keywords and syntax won't get you far.