How about this? 
#!/usr/bin/perl

use strict;
use Data::Dumper;

local $/ = ">";
my %hash;
while (<DATA>) {
    s/>//;
    s/(^SEQ\d)//;
    $hash{">". $1} = $_ if ( defined( $1 ));
}

print Dumper(\%hash);

__DATA__
>SEQ1
-----I--RL--AAIDVDG-NLT----------D--R--D-RL-ISTKA-IESIRS--A-
-E-K--------K-GLT-VSL----LS------GN-V----I-PVV---YA-L------K
IF---------------L-----GINGPVF------------------------------
>SEQ2
-MKI----KA--ISIDIDG-TIT------YPN-R-------MIHEK--A-LEAIRR--A-
-E-S--------L-GIP-IML----VT------GN-T----V-QFA---EA-A------S
IL---------------I-----G----TS-----------------GP-VV--------
>SEQ3
--KI----KA--ISIDIDG-TIT------YPN-R-------MIHEK--A-LEAIRR--A-
-E-S--------L-GIP-IML----VT------GN-T----V-QFA---EA-A------S
IL---------------I-----G----TS-----------------GP-VV--------
---AE--D------GG---A---------------------------------------I

[download]

In reply to Re: splitting data advice requested by bichonfrise74
in thread splitting data advice requested by Angharad

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.