The code is pretty much the same except that the second page data gets new lines inserted in front of the id codes and we do a little clean up to remove white space at the ends of lines:

use strict;
use warnings;
use Text::CSV;

my $page1 = <<PG1CSV;
512.45,c100
6734, c200
5653.2, c300
PG1CSV

my $csv = Text::CSV->new();
my %idData;

open my $pg1In, '<', \$page1;

while (my $row = $csv->getline($pg1In)) {
    s/^\s+|\s+$//g for @$row;
    $idData{$row->[1]}{size} = $row->[0];
    $idData{$row->[1]}{name} = '-- missing --';
}

close $pg1In;

my $page2 = <<PG2CSV;
c100, Joe Shmo c200, Jack Black c300, Cinderella c400, Barack Obama c5
+00, Cruella Deville 
PG2CSV

$page2 =~ s/\b(?=\w+,)/\n/g; # Insert newlines in front of id codes

open my $pg2In, '<', \$page2;

while (my $row = $csv->getline($pg2In)) {
    next if !$row->[0]; # Skip blank lines
    
    s/^\s+|\s+$//g for @$row;
    $idData{$row->[0]}{name} = $row->[1];
    $idData{$row->[0]}{size} //= '-- missing --';
}

close $pg2In;

for my $id (sort keys %idData) {
    print "$id: $idData{$id}{name} size $idData{$id}{size}\n";
}
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Prints:

c100: Joe Shmo size 512.45
c200: Jack Black size 6734
c300: Cinderella size 5653.2
c400: Barack Obama size -- missing --
c500: Cruella Deville size -- missing --
[download]