You have a lot of options in your approach, here. I humbly offer my somewhat more Perl-ish approach to this problem.

The basis of what I did is to get rid of hard-coded fields and allow you to specify them near the top of the script in @fields, building a regexp to capture them in the header line. I relaxed the usage of the program as well, to allow for specifying a filename on the command line, or, if none is specified, accepting input from STDIN. You can easily change the output format by modifying the final for (sort keys %inv) { ... } loop, and you can change how records are grouped by changing the key() function to suit your tastes. In both cases, I tried to stay with what you had, since I don't know what you want to ultimately do with the data.

use strict;
use warnings;
use 5.12.00;

die "usage: $0 [filename]\n" if (@ARGV > 1);

# Set required fields and determine fixed widths
my @fields = qw/Part Shape Color Size/;
my $re;
$re .= qr/(?<$_>$_\s*)/ for @fields;

my $header = <>;
die "Header must match " . join(' ', @fields) unless ($header =~ /^$re
+$/);
my $expected_len = length($header);

my $tmpl = join(' ', map { "A[".length($+{$_})."]" } @fields);

my %inv; # Inventory; $inv{key(%rec)}

while (<>) {
    if (length != $expected_len) {
        die sprintf("Got length of %d, expecting %d", length, $expecte
+d_len);
    }
    my @rec = map { /(.+?)\s*$/ } unpack $tmpl; # Get (trimmed) record
+s
    my %rec = map { $_ => shift @rec } @fields;

    # Add the part to our inventory
    push @{$inv{key(%rec)}}, $rec{Part};
}

# Now print out the inventory in the desired format
for (sort keys %inv) {
    say join(' ',@{$inv{$_}}) . " - $_ - " . scalar @{$inv{$_}}; 
}

# Our custom hashing function for inventory items. Expects %rec argume
+nt
sub key { my %rec = @_; $rec{Shape} . $rec{Color} . $rec{Size}; }
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