Judging by the code I see at PerlMonks and in the various CPAN source I have looked at, post-fix statements are pervasive among experienced Perl users. I think partly this is true because they break through the clutter of more conventional languages to make "simple things simple" and clear.

It is somewhat the difference between writing short comprehensible sentences in prose and writing longer tortuous incomprehensible meandering sentences that are harder to parse and harder to understand. Perl encourages short comprehensible sentences.

For me, the choice between:

consequence() if condition(); # or
condition() and consequence();
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And the choice between:

consequence unless condition(); # or
condition() or consequence();
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Is mostly a question of front-loading. What part of the statement seems the most important in its context? If we're just trying to avoid warnings about undef values, then adding if defined $foo to the end of a statement seems less obnoxious than putting the condition up front. But if we're going to do a lot of processing on $line, then my $line = <$fh> or return; is probably clearer than return unless my $line = <$fh>;.

toby döt ink

You want to "write clearly" and "say what you mean"? Great! Postfix conditionals help you to do that, by mirroring the syntax of common English sentences, such as "I would understand the use of post-fix if construct [implied "if"] in a loop to specify exit condition.", instead of having to rewrite your plain-English thoughts to force the conditional to always be stated first.