From your description, this should be close to what you want:

my $newseq = do { local $/; <INFILE> };

$newseq =~ tr/01//cd;
$newseq =~ tr/0/a/;
$newseq =~ s/(1+)/ chr length( $1 ) + ord 'a' /eg;

print "$_\n" for $newseq =~ /.{0,80}/g
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