while (my $line1 = <$file2>){ 
        
  chomp $line1; 
  
  # Item,Sink,Buffer,Cell,Slew,Path,Violation,Area
  my @f = split /,/, $line1 ; 
  my $item = $f[0];
  my $slew = $f[4];
  next if $item eq 'Item'; # skip header
        
  if (defined $file1Hash{$item}) {
    my $period = $file1Hash{$item};
    my $pc = sprintf "%.10f",100*$slew/$period unless ($period==0);
    print $outfile_1 join ',',$line1,$period,$pc."\n";        
  } 
}
[download]

use strict;
use warnings;
use Data::Dumper;

my $file1 = "an_en11,200.00,{0 133},{ }
br_gh13,140.09,{0 59},{ }
ce_oy74,300.05,{0 230},{int_43}
dt_pp50,200.11,{0 122},{ }
er_tk02,305.47,{0 220},{ }
ef_yb41,200.05,{0 233},{ }";

open my $FH1, "<", \$file1 or die "cannot open $file1";
my %file1Hash;
while (my $line = <$FH1>) {
    my ($item, $period) = split /,/, $line;
    $file1Hash{$item} = $period;
}
close $FH1;
print Dumper \%file1Hash;

my $file2 = "dt_pp50,0,0,2,0.000,0.000,0,0.000
er_tk02,0,2,3,0.002,0.004,0,0.001
ef_yb41,0,1,5,0.000,0.000,0,0.000";

open my $FH2, "<", \$file2 or die "cannot open $file2";
while (my $line = <$FH2>) {
    chomp $line;
    my ($item, $slew) = (split /,/, $line)[0,4];
    next unless $file1Hash{$item}; # skip the line if the period is no
+t defined or if it is 0
    my $skew = 100 * $slew / $file1Hash{$item};
    printf "%s,%d,%.8f\n", $line, $file1Hash{$item}, $skew;
}
[download]

And an sample execution (at the command line):

HTH.