Thanks to all who gave a hint into what direction I should work. All great examples given as well. Finally I've got something working which I'll post below. I'm still a bit puzzled why I need to put the closing bracket of the Foreach loop higher by using real datafiles

} # real datafiles
    my ($basename) = ( $filename =~ m/^(\S+)\.txt/ ) or warn;
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as opposed to when I make use of the sample data

    printf "        D + C: \t%15s%16s\n",
      &big_money(
        ( $saldi{$filename}{'"D"'} * -1 ) + ( $saldi{$filename}{'"C"'}
+ ) ),
      &big_money($deltaSum);
} #sample
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but it works! Script below makes use of sample data. And only has a problem when irregular pattern appears to be somewhere in the middle of the file (which should never be the case).
It thaugt me a great deal about hashes and how you need to refer to the data.
thanks all, Gert

#!/usr/bin/perl -w
use strict;
use diagnostics;
use vars qw ! %saldi $filename !;

#chdir  # real code
#  '/Users.......'    # real code
#  or die "Cannot open : $!";                                #real cod
+e
#foreach my $filename ( glob "*.txt" ) {    # real code       
#    open FILE, '<', $filename or warn "Can't open FILE: $!";#real cod
+e
#    while (<FILE>) {                                        #real cod
+e
#        chomp;                                              #real cod
+e
#        next unless $_;                                     #real cod
+e

while (<DATA>) {    # Required for sample code
     chomp;          # Required for sample code
     next unless $_; # Required for sample code
     $filename = $1, next if /(\w+\.txt)$/; # Required for sample code

        my @fields = ( split /,/, )[ 0, 1, 2, 3, 4 ];
        my ( $account, $currency, $year, $dofc, $amount ) = @fields;
        next unless $dofc && $amount;
        $saldi{$filename}{$dofc} += $amount;    # file name as primary
+ key
    }

    my $dSum     = 0;
    my $cSum     = 0;
    my $deltaSum = 0;

    foreach $filename ( keys %saldi ) {
        $cSum += $saldi{$filename}{'"C"'};
        $dSum += $saldi{$filename}{'"D"'} * -1;

        my $delta =
          ( $saldi{$filename}{'"D"'} * -1 ) + $saldi{$filename}{'"C"'}
+;
        $deltaSum += $delta;
 #} real   
    my ($basename) = ( $filename =~ m/^(\S+)\.txt/ ) or warn;
    printf "$basename\n";

    printf "\tD sum: \t%16s",
      &big_money( $saldi{$filename}{'"D"'} * -1 ) . "\n";
    printf "\tC sum: \t%16s", &big_money( $saldi{$filename}{'"C"'} ) .
+ "\n";

    printf "        D + C: \t%15s%16s\n",
      &big_money(
        ( $saldi{$filename}{'"D"'} * -1 ) + ( $saldi{$filename}{'"C"'}
+ ) ),
      &big_money($deltaSum);
} #sample
#    close FILE or warn "Can't close FILE: $!";    # real code
#}    # real code

sub big_money {    #Learning Perl
    my $number = sprintf "%.2f", shift @_;

    #Add one comma each time though the do-nothing loop
    1 while $number =~ s/^(-?\d+)(\d\d\d)/$1,$2/;

    #Put the dollar sign in the right place
    $number =~ s/^(-?)/$1/;
    $number;
}

__DATA__
Data_1.txt
394,eur,2006,"D",18.20
394,eur,2006,"D",22
394,eur,2006,"C",25
394,eur,2006,"C",16
Data_2.txt
494,eur,2006,"C",25
494,eur,2006,"D",79
494,eur,2006,"D",79
494,eur,2006,"D",79
494,eur,2006,"C",100
494,eur,2006,"C",1
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