Here's how I'd do it:

use strict;
use Data::Dumper;

my $delimiter = "|";  # delimiter for multiple tags
my %data;
while(my $line = <DATA>) {
  chomp $line;
  $line =~ s/^(.*?):\s*//; # remove leading number and colon, and any 
+whitespace
  
  my $rec = {};
  my $partno;
  for my $field ( split(/,/, $line) ) {
    my ($key, $value) = split(/=/, $field);
    if ($key eq 'partnum') {
      $partno = $value;
    } else {
      $rec->{$key} = $value;
    }
  }

  
  if ( defined $data{ $partno } ) {
  
    if ( $data{$partno}{'tags'} ) {
      $data{$partno}{'tags'} .= $delimiter . $rec->{'tags'};
    } else {
      $data{$partno}{'tags'} = $rec->{'tags'};
    }
    
    $data{$partno}{'quantity'} += $rec->{'quantity'};
    
    unless ($data{$partno}{'description'} eq $rec->{'description'}) {
      warn "Multiple descriptions for $partno ! \n";
    }
    
  } else {
    $data{ $partno } = $rec;
  }
  
}

print Dumper(\%data), "\n";

__DATA__
413: partnum=2204133000,description=PRESS GAUGE,quantity=1.0000,tags=P
+I-412
414: partnum=2202261000,description=THERMOWELL,quantity=2.0000,tags=
415: partnum=2201176000,description=THERMOMETER,quantity=2.0000,tags=
581: partnum=2204227002,description=TEMP TRANSMITTER,quantity=1.0000,t
+ags=TE/TT-102
582: partnum=2201176000,description=THERMOMETER,quantity=3.0000,tags=T
+I-100 TI-101 TI-200
576: partnum=2204133000,description=PRESS GAUGE,quantity=1.0000,tags=P
+I-400
[download]

Notes:

• I use the part number as the main hash key.
• I use 'defined' to check for duplicates.
• If I find a duplicate, I append the tag, if another tag exists. (it would be better to keep the tags in an array, to avoid having to split this list later)
• I separate tags using the character in $delimiter, so you can make it whatever you like.
• If I find a duplicate, I combine the quantities, and sanity check the descriptions (should be the same?) - consider this optional.

I get this output:

$VAR1 = {
          '2201176000' => {
                            'quantity' => 5,
                            'description' => 'THERMOMETER',
                            'tags' => 'TI-100 TI-101 TI-200'
                          },
          '2204133000' => {
                            'quantity' => 2,
                            'description' => 'PRESS GAUGE',
                            'tags' => 'PI-412|PI-400'
                          },
          '2202261000' => {
                            'quantity' => '2.0000',
                            'description' => 'THERMOWELL',
                            'tags' => ''
                          },
          '2204227002' => {
                            'quantity' => '1.0000',
                            'description' => 'TEMP TRANSMITTER',
                            'tags' => 'TE/TT-102'
                          }
        };
[download]

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