Since your data is in comma separated format, using an absolute offset to extract the number you want doesn't make sense. You can use the Text::CSV module to parse the data into separate fields:

use Text::CSV;

my $file = "trace.csv";
my $file1 = "rgextract.txt";

open my $io, '<', $file or die "$file: $!";
open my $out, '>>', $file1 or die "$file1: $!";

my $csv = Text::CSV->new({ binary => 1, eol => $/ });
while (my $row = $csv->getline ($io)) {
    my $number = $row->[3];
    print $out "$number\n";
}
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With the 4 data lines you posted, this will create the rgextract.txt file:

114
118
114
114
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