Split the header line into an array before entering the loop, and then use that to build an AoHs:

use strict;
use warnings;

my $file = "/tmp/index.csv";
open (FL, "< $file");
my @fieldnames = split ';', <FL>; ##
my @data;
while ($line = <FL>){
    my %temp; @temp{ @fieldnames } = split ';', $line;
    push @data, \%temp;
}

## print the 'Forex' field of the 124th line (array runs from 0).
print $data[ 123 ]{ Forex };
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