A self-contained example solution using the HoH you were thinking of.

johngg@shiraz:~/perl/Monks > perl -Mstrict -Mwarnings -E '
open my $file1FH, q{<}, \ <<__EOD1__ or die $!;
App1,4,Perl
App2,8,Java
App3,8,Java
App4,4,PHP
App5,8,C#
__EOD1__

my %apps;
while ( <$file1FH> )
{
    chomp;
    my( $key, $mem, $lang ) = split m{,};
    $apps{ $key }->{ mem }  = $mem,
    $apps{ $key }->{ lang } = $lang;
}

close $file1FH or die $!;
open my $file2FH, q{<}, \ <<__EOD2__ or die $!;
App1,1.5,2
App2,2.5,4
App3,2.8,4
App4,2.8,2
App5,2.8,2
__EOD2__

while ( <$file2FH> )
{
    chomp;
    my( $key, $cpu, $cores ) = split m{,};
    $apps{ $key }->{ cpu }   = $cpu;
    $apps{ $key }->{ cores } = $cores;
}

say for
   sort { $apps{ $a }->{ mem } <=> $apps{ $b }->{ mem } }
   grep { $apps{ $_ }->{ cores } eq q{2} }
   sort keys %apps;'
App1
App4
App5
[download]

I hope this is useful.

Update: Corrected spelling mistake and added the following note:-

The reason I sort the keys before passing them into the grep is that the order returned by the keys function is essentially random. Since Perl's sort is "stable" it would not change the order in which "App1" and "App4" appeared as they have the same memory value so a result of

App4
App1
App5
[download]

could happen.