I generally try to avoid operating on @_ and $_ wherever possible, to avoid confusion and pitfalls like what you may be experiencing. Here's some code I whipped up that I think does the operation you desire but in a slightly different way. 
#!/usr/bin/perl
use strict;
use warnings;

my $var = '';
my @array = ();

foreach my $line (<DATA>) {

    chomp $line;
    @array = split(/\t/,$line);
    $var = $array[0]; #<- not necessary, but wanted to mimic OP's code
    print "$var\n";

}

exit;

__DATA__
1    4    8    9
23    432    23    21
98    87    4    32

[download]
Does that help any?

In reply to Re: Yet another array problem by kwaping
in thread Yet another array problem by Angharad

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