while (defined( my $line = <UNIPROT> )) {
	if ($line =~ /^(AC|OS|OX|ID|GN)\s+(.*)/) {
		print "<$1> $2\n";
	}
}

__END__

<ID> ARF1_PLAFA              Reviewed;         181 AA.
<AC> Q94650; O02502; O02593;
<GN> Name=ARF1; Synonyms=ARF, PLARF;
<OS> Plasmodium falciparum.
<OX> NCBI_TaxID=5833;

##</code><code>##

#!/usr/bin/env perl
use warnings;
use strict;

my $filename = "uniprotfile";
open my $ufh, "<", $filename
	or die "open $filename: $!";

while (<$ufh>) {
	chomp;
	my ($id,$content) = /^(AC|OS|OX|ID|GN)\s+(.*)/
		or next;
	if ($id eq 'AC') {
		my ($first) = $content=~/^([^;]+)/
			or die "couldn't parse '$content'";
		print "AC: $first\n";
		...
	}
	elsif ($id eq 'OS') {
		...
	}
	...
}