$ cat pm_1193237_input.fasta
>qwerty|111|222|999
AAAAAAAAAA
>asdfgh|333|444|888
CCCCCCCCCC
>zxcvbn|555|666|777
GGGGGGGGGG
>plokmi|777|888|666
TTTTTTTTTT
>qwerty|111|222|555
AAAAAAAAAA
>asdfgh|333|444|444
CCCCCCCCCC
>zxcvbn|555|666|333
GGGGGGGGGG
>plokmi|777|888|222
TTTTTTTTTT

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;
use autodie;

my $infile = 'pm_1193237_input.fasta';

{
    my $out_fh;

    open my $in_fh, '<', $infile;

    while (<$in_fh>) {
        my $pos = index $_, '|';

        if ($pos == -1) {   # Sequence
            print $out_fh $_;
        }
        else {              # Header
            print { get_fh(\$out_fh, substr $_, 1, $pos - 1) } $_;
        }
    }

    close $in_fh;
}

close_out_fhs();

{
    my %fh_for;

    sub get_fh {
        my ($fh, $name) = @_;

        unless (exists $fh_for{$name}) {
            open $fh_for{$name}, '>', gen_fname($name);
        }

        $$fh = $fh_for{$name};
    }

    sub close_out_fhs { close $_ for values %fh_for }
}

sub gen_fname { 'pm_1193237_output_' . $_[0] . '.fasta' }

##</code><code>##

$ cat pm_1193237_output_qwerty.fasta
>qwerty|111|222|999
AAAAAAAAAA
>qwerty|111|222|555
AAAAAAAAAA
$ cat pm_1193237_output_asdfgh.fasta
>asdfgh|333|444|888
CCCCCCCCCC
>asdfgh|333|444|444
CCCCCCCCCC
$ cat pm_1193237_output_zxcvbn.fasta
>zxcvbn|555|666|777
GGGGGGGGGG
>zxcvbn|555|666|333
GGGGGGGGGG
$ cat pm_1193237_output_plokmi.fasta
>plokmi|777|888|666
TTTTTTTTTT
>plokmi|777|888|222
TTTTTTTTTT