As Javafan said there is no reason to read more than one number from the file at a time. To do this you might use (for example) the module File::Stream which allows you to read your array file one number at a time with 
use File::Stream;
my $stream = File::Stream->new($filehandle);
$/ = qr/\s+/;
...
$nextnum= <$stream>;

[download]

UPDATE: Here is an example implementation that seems to work (but with harcoded n and m)

#!/usr/bin/perl
use warnings;
use strict;

use File::Stream;

my $m=3;
my $n=4;
my $filehandle;

open ($filehandle, '<',  "testfile") || die "can't open the testfile";


my $stream = File::Stream->new($filehandle);
$/ = qr/\s+/;


my @a;
my @ia;
my @ja;

my $nnz=1;

my $x=0;
my $y=0;
my $value;
while ($value=<$stream>) {
    $value=~s/\s//g; #because chomp doesn't work with File::Stream
    $x++;
    if ($y==0 or $x>$n) { 
    $x=1; $y++;
    push @ia,$nnz;
    }
    if ($value) {
    push @a, $value;
    push @ja, $x;
    $nnz++;
    }
}
push @ia,$nnz;

if ($y!=$m and $x!=$m) {
    print "Wrong number of values in array data\n";
}

print 'A= [ ', join(' ',@a),"]\n";
print 'IA= [ ', join(' ',@ia),"]\n";
print 'JA= [ ', join(' ',@ja),"]\n";

[download]

UPDATE2: Check out kennethk's version below. He reads one line at a time. If you can guarantee that n is lower than lets say 10^8 for any matrix you have to process, his version is probably a lot faster. But if you can't rule out matrices with bigger n, something like above has to be done to read even lines in smaller chunks.

In reply to Re: Memory Efficient Sparse Matrix Handling by jethro
in thread Memory Efficient Sparse Matrix Handling by neversaint

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