Why doesn’t print output anything on each iteration of a loop when I use sleep?

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The real issue has nothing to do with sleep, but rather that…………

You are Suffering from Buffering. The link provided takes you to an excellent article from The Perl Journal circa 1998 from Marc Jason Dominus (the author of Higher-Order Perl). The article may be over a decade old, but the topic is as relevant today as it was when he wrote it.

Others have explained the $| = 1; technique. I would add to those comments that in the predominant thinking of the Perl community seems to be that $| = 1 is preferable over $|++ simply because it is clearer in its meaning. I know, autoincrement is pretty simple too, but does everyone who will ever look at your code know $|‘s behavior when ++ or -- are applied (without looking it up in perlvar). I happen to also prefer to localize any modification of Perl’s “special variables” so that the effects are not washing over into other portions of code that may not play nice with a particular change to default Perl behavior. So that being the case, I would write it as:

use strict;
use warnings;

{
    local $| = 1;
    for ( 1 .. 20 ) {
        print '.';
        sleep 1;
    }
}

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