>Record 1
AGTCTAGTCAT
CATCATAAGAT
CATCAATCACA
>Other Record
ATGAACAGCAG
ATGAAGAATGG
ATAG

##</code><code>##

% perl -lne 's/>//?$s=$_:$s{$s}.=$_;\
> END{ <do smthng w/ %s> }' \
> huge.fasta massive.fasta humongo.fasta 

##</code><code>##

% perl -MStorable -lne 's/>//?$s=$_:$s{$s}.=$_;\
> END{ store \%s "for_later" }' \
> huge.fasta massive.fasta humongo.fasta