Change print to return, and it works.

use warnings;
use strict;

my $string = "this is a test \$placeholder";

$string =~ s/\$placeholder/test_sub()/e;

print $string;


sub test_sub
{
  return "hello there";
}
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If you really need to capture the output of what a sub prints, that's a whole other problem. I was doing something like that in Use of uninitialized value in open second time but not first., and there's more discussion with it about other ways to do it.