I think this code is shorter but still does what you want.

use strict;

my @fasta;

for my $acc (@accession)
{
    next unless /_(\d+);/

    my id   = $1;
    my $seq = get( "http://us.expasy.org/cgi-bin/get-sprot-fasta?$id";
    unless (defined $seq)
    {
        print "Couldn't find id $id\n";
        next;
    }
    push @fasta, $seq;
}
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