my $cur_tag;
my %lines_for;

while(<>) {
    chomp;
    my ($tag, $line) = split /\s+/, $_, 2;
    $cur_tag = $tag || $cur_tag 
        or die "First input line has no tag";
    last if $cur_tag eq 'ER';
    push @{$lines_for{$cur_tag}}, $line;
}
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Makeshifts last the longest.