#!/usr/bin/perl -w

@files = <*.csv>;
foreach $file (@files) {
  open(FH, $file) || die("Error: Cannot open file $file for reading.");
  my @dt = ($file =~ /^(\w+).(\d{6})\.csv$/);
  while (<FH>) { 
    print "@dt[0] $_\n";
  }
  close(FH);
}

my %files = map {/^(\w+).\d{6}\.csv$/; $_ => $1} <*.csv>;
print Dumper \%files;
foreach my $file (keys %files) {
  open(FH, $file) || die("Error: Cannot open file $file for reading.");
  while (<FH>) { 
    print "$files{$file} $_\n";
  }
  close(FH);
}