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

my $page1 = <<PG1CSV;
1,23
2,10
3,23
PG1CSV

my $page2 = <<PG2CSV;
1,younggrasshopper13
2,GrandFather
4,Mr. Unknown
PG2CSV

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

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

while (my $row = $csv->getline($pg1In)) {
    $idData{$row->[0]}{size} = $row->[1];
    $idData{$row->[0]}{name} = '-- missing --';
}

close $pg1In;

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

while (my $row = $csv->getline($pg2In)) {
    $idData{$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";
}