Hi! You have a few things that could be improved in your Perl (don't use bareword file handles) and a few things in your expression as well:

#!/usr/bin/perl

use warnings;
use strict;

open my $fh, '<', 'uniprotfile' or die "Can't open uniprot file: $!";

while (defined(my $line = <$fh>)) {
    chomp $line;
    if ($line =~ /^(AC|OS|OX|ID|GN)\s+(.*)/) {
        print  $1, $2, "\n";
    }
}
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Instead of repeating yourself in the expression, simplify it a bit.