It is so slow on large files because for each matching record, you loop through all 80,000 lines; So, if you had had 4000 matching records, you would have 4000 * 80000 = 320,000,000 iterations. There must be a better method I think.

And I don't know if you can 'tie' the same file (as an array) while opening it for reading (both at the same time).

Note that I set the input record separator, $/, to ---- lsattr, (with a space following lsattr), to read a record at a time.

Not seeing more sample data, I made a guess at what might work and it did work with your sample data. But again, it's difficult to tell.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $base = $ARGV[0] or die "Must supply a filename to open. $!";

open my $fh, "<", $base or  die "Unable to locate file: $!\n";

my %data;

{
    local $/ = "---- lsattr ";
    while (<$fh>) {
        chomp;
        next unless /^-El\s+(\S+)/;
        my $vg = $1;
        
        next unless /^label\s+(\S+)/m;
        my $label = $1;
        next if $label eq "None";
        
        next unless /^lvserial_id\s+(\S+)/m;
        my $name = $1;
        
        next unless /^size\s+(\d+)/m;
        my $size = $1;
        
        @{ $data{ $vg } }{ qw/ label name size / } = ($label, $name, $
+size);
        
        #print "VG: $vg : MOUNT: $label : LV_name: $name : SIZE: $size
+\n";
    }
}

print Dumper \%data;
[download]

Update: The data structure created above will only work if there is only 1 record for each sought key, ($vg). If there is more than 1 record with the same key, the data structure will only contain the last fields parsed from the file. It will silently give you incorrect results.

That said, I would need to know more about your file to be able to suggest a suitable data structure.