Right, but you don't have to presize it. Just push entries onto it as you parse them.

For instance, to invent a stupid-simple data format, and demonstrate parsing it and doing a thing or two with the data once parsed:

#!/usr/bin/env perl5
use strict;
use warnings;

my @clients;
my %tmp;

while(<DATA>)
{
    chomp;

    # Assume a format of "Type Data"
    my ($type, $data) = split / /;

    # If it's a MAC address, save off our pre-existing record (if any)
+,
    # and start a new one.
    if($type eq "MAC")
    {
        if(keys %tmp)
        {
            # Have to unroll a copy here, otherwise the next line empt
+ies
            # it out.
            push @clients, { (%tmp) };
            %tmp = ();
        }

        $tmp{MAC} = $data;
    }

    # RSSI and SNR just get saved
    $tmp{RSSI} = $data if $type eq "RSSI";
    $tmp{SNR} = $data  if $type eq "SNR";
}

# Save the 'last' one if we fell off the end
push @clients, \%tmp if keys %tmp;

# Show it
use Data::Dumper; print Dumper \@clients;

# Find a given entry
sub showmacsnr
{
    my $mac = shift;
    my @ba = grep { $_->{MAC} eq $mac } @clients;

    if(@ba)
    {
        my $ent = $ba[0];
        print "MAC $mac has SNR $ent->{SNR}\n";
    }
    else
    {
        print "Can't find MAC $mac\n";
    }
}

showmacsnr("ba:98:76:54:32:10");
showmacsnr("thi:is:is:nt:re:al");


# Sample data
__DATA__
MAC 01:23:45:67:89:ab
RSSI 12
SNR 18
MAC ba:98:76:54:32:10
RSSI 7
SNR 3
[download]

So you end up with output like:

% ./tst.pl 
$VAR1 = [
          {
            'RSSI' => '12',
            'SNR' => '18',
            'MAC' => '01:23:45:67:89:ab'
          },
          {
            'SNR' => '3',
            'RSSI' => '7',
            'MAC' => 'ba:98:76:54:32:10'
          }
        ];
MAC ba:98:76:54:32:10 has SNR 3
Can't find MAC thi:is:is:nt:re:al
[download]