use warnings;
use strict;

#  is one way to get the file names
#  I think here we can just use glob() if you are at Perl 5.10
#  my $source_dir = "C:/convs";
#  opendir (DIR, $source_dir) || die "unable to open $source_dir $!";
#  @files = grep{m/conv\d+\.txt/}readdir DIR;

my $source_dir = "C:/convs";
my @files = glob("$source_dir/conv*.txt");

foreach my $file (@files)
{
   open (IN, "$source_dir/$file") 
                    || die "unable to open $source_dir/$file $!";
   open (TEMP, "$source_dir/$file.tmp" 
                    || die "unable to open $source_dir/$file.tmp $!;
   while (<IN>) 
   {
       s/^.*Mina olen.{1561}//s; #/s allows "." to match newline
                                 #I'm not sure that it is needed here.
       print TEMP $_;
   }
   close TEMP || die "$!"; #unlikely to fail (file is "open")
   close IN   || die "$!";
   unlink ("$source_dir/$file") || die "$!";
   rename ("$source_dir/$file.tmp", "$source_dir/$file") || die "$!";
}