Assuming your data is valid, please consider:

use feature ":5.14";
use warnings FATAL => qw(all);
use strict;
use Data::Dump qw(dump pp);

my @f1 = split /\n/, <<'END';
2010-nhw-es-ea-285, 2012-10-10
aab.external.audit.-.bvr.verif.externes, 2012-02-22
aashee.aziz, 2012-01-27
abbas.rangwala, 2012-01-27
abbie.gardner, 2012-09-10
ab.border-frontiere, 2012-08-11
abdool.yasseen, 2012-01-31
abdullah.aboukarr, 2012-08-10
abdullahi.sheikh, 2012-02-28
END

my @f2 = split /\n/, <<'END';
aaron.northway, CHIEF FINANCIAL OFFICER BRANCH 
aashee.aziz, HEALTH PRODUCTS AND FOOD BRANCH 
abbey.klugerman, HEALTHY ENVIRONMENTS AND CONSUMER SAFETY BRANCH 
abby.hoffman, STRATEGIC POLICY BRANCH 
abderrahim.boussanni, CHIEF FINANCIAL OFFICER BRANCH 
abdiaziz.nur, HEALTHY ENVIRONMENTS AND CONSUMER SAFETY BRANCH 
abdool.yassin, HEALTH PRODUCTS AND FOOD BRANCH 
abdoulie.manjang, N/A  
abdullah.hassen, HEALTH PRODUCTS AND FOOD BRANCH 
abdullah.patel, REGIONS AND PROGRAMS BRANCH
END

my (%f1, %f2); 

for(@f1)
 {my ($username, $date) = split /,\s*/;
  $f1{$username} = $date;
 }

for(@f2)
 {my ($fullname, $branch) = split /,\s*/;
  $f2{$fullname} = $branch;
 }

my %names = map {($_, 1)} (keys %f1, keys %f2);

say "$_, ", ($f1{$_}//'undefined'), ", ", ($f2{$_}//'undefined') for s
+ort keys %names;
[download]

Produces:

2010-nhw-es-ea-285, 2012-10-10, undefined
aab.external.audit.-.bvr.verif.externes, 2012-02-22, undefined
aaron.northway, undefined, CHIEF FINANCIAL OFFICER BRANCH 
aashee.aziz, 2012-01-27, HEALTH PRODUCTS AND FOOD BRANCH 
ab.border-frontiere, 2012-08-11, undefined
abbas.rangwala, 2012-01-27, undefined
abbey.klugerman, undefined, HEALTHY ENVIRONMENTS AND CONSUMER SAFETY B
+RANCH 
abbie.gardner, 2012-09-10, undefined
abby.hoffman, undefined, STRATEGIC POLICY BRANCH 
abderrahim.boussanni, undefined, CHIEF FINANCIAL OFFICER BRANCH 
abdiaziz.nur, undefined, HEALTHY ENVIRONMENTS AND CONSUMER SAFETY BRAN
+CH 
abdool.yasseen, 2012-01-31, undefined
abdool.yassin, undefined, HEALTH PRODUCTS AND FOOD BRANCH 
abdoulie.manjang, undefined, N/A  
abdullah.aboukarr, 2012-08-10, undefined
abdullah.hassen, undefined, HEALTH PRODUCTS AND FOOD BRANCH 
abdullah.patel, undefined, REGIONS AND PROGRAMS BRANCH
abdullahi.sheikh, 2012-02-28, undefined
[download]