use warnings;
 use strict;
 
 my @fh; # Store filehandles
 
 open $fh[@fh], '<', $_ or die "Can't open `$_': $!" for <1039418/*.txt>;
 
 while (1) {
     no warnings qw/uninitialized io/;
     my $line;
     $line .= <$_> // "\n" for @fh;
     last if $line =~ /^\n*$/; # All files finished or blank
     $line =~ s/\n(?!\z)/,/g;  # Delimit with commas, except for last \n
     print $line;
 }