Just a quick piece of code that seems to be doing what you want: 
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]
Note that I embedded the input data into the code for simplicity of the test.

And an sample execution (at the command line): 

$ perl -e '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 
+not defined or if it is 0
>     my $skew = 100 * $slew / $file1Hash{$item};
>     printf "%s,%d,%.8f\n", $line, $file1Hash{$item}, $skew;
> }'
$VAR1 = {
          'dt_pp50' => '200.11',
          'an_en11' => '200.00',
          'br_gh13' => '140.09',
          'er_tk02' => '305.47',
          'ce_oy74' => '300.05',
          'ef_yb41' => '200.05'
        };
dt_pp50,0,0,2,0.000,0.000,0,0.000,200,0.00000000
er_tk02,0,2,3,0.002,0.004,0,0.001,305,0.00065473
ef_yb41,0,1,5,0.000,0.000,0,0.000,200,0.00000000

[download]
Also note that I used split to keep closer to your code, but it would make sense to use Text::CSV_XS or Text::CSV as recommended earlier by other monks, especially if your input data gets more complicated.

HTH.

In reply to Re^3: How to add a new column into my csv file? by Laurent_R
in thread How to add a new column into my csv file? by chaney123

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