..
my $list_file = '/path/to/list.txt';

open my $read, '<', $list_file or die "cannot read from'$list_file'!";

while (<$read>){  # it populates $_ is the same of: defined $_ = <$read>
  chomp;
  my $ret = process ($_) if -e "./$_";
  if ($ret){print "$_ succesfully processed\n"}
  else{print "Warning: '$_' was not processed!\n"}
}

sub process{
 my $file = shift;
 open ... or return 0
 # read and do something
 close ..
 # implicitly return the last thing: if close succeded is 1 
}