In BASIC-PLUS, which is where Larry stole the IF/WHILE/UNTIL/UNLESS suffix forms, you can nest them, however. So, this would be legal:

print $_ if $_ % 2 while <INPUT>; # not legal in Perl, but in pseudo-B
+ASIC-PLUS
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However, in Perl you're forced to write that in two pieces somehow:

$_ % 2 and print $_ while <INPUT>;
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or

while (<INPUT>) { print $_ if $_ % 2 }
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I believe Larry thought the nested stacked suffix operations were hard to parse, and I agree with him.

-- Randal L. Schwartz, Perl hacker