The DBD::AnyData module handles both CSV and Fixed Length formats. Here would be the entire script to convert from one to the other:

#!/usr/bin/perl -w
use strict;
use DBI;
my $pattern = {pattern=>'A6 A9 A8 A5 A3'}; # widths of cols
my $dbh=DBI->connect('dbi:AnyData(RaiseError=1):');
$dbh->ad_catalog('T1','CSV','yourfile.csv');
$dbh->ad_catalog('T2','Fixed','yourfile.fix',$pattern);
$dbh->do("CREATE TABLE T2 AS SELECT * FROM T1");
__END__
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If you don't like SQL, the AnyData module will let you do the same thing with a tied-hash interface.