Path::Tiny provides a nice API at a slightly higher level than readdir and friends.

Edit: tweaking to put each inputs' corresponding output into a separate file.

use Path::Tiny qw( path );
my $src_dir = path( q{/home/blah/blah/blah/yakety/smakety} );
my $out_dir = path( q{/home/blah/blah/blah/output/} );

for my $txt_file_in_src ( $src_dir->children( qr{ \.txt \z }x ) ) {
  open( my $infh, q{<}, $txt_file_in_src )
    or die qq{Can't open '$txt_file_in_src': $!\n};
  my $outfile = $out_dir->child( $txt_file_in_src->basename . q{_out.t
+xt} );
  open( my $outfh, q{>}, $outfile )
    or die qq{Can't create '$outfile': $!\n} );

  ## Alternately use Path::Tiny helper
  ## my $infh = $txt_file_in_src->openr;
  ## my $outfh = $outfile->openw;

  ## process source file reading from $infh writing to $outfh . . .
}
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