You don't really need the arrays @p, @q and @i, you could just do this:

my %f;
for my $file ( @files ) {
  open my $IN, '<', $file or die "Cannot open '$file' because: $!";
  while ( <$IN> ) {
    /^(.*)\t(.*)$/ && $f{ $1 } += $2;
  }
}

open my $OUT, '>', $outpath or die "Cannot open '$outpath' because: $!
+";

for my $key ( sort { $f{ $b } <=> $f{ $a } } keys %f ) {
  print $OUT "$key\t$f{$key}\n";
}
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