Sure you can. As exemplified in this simple command-line (one-liner) script; 
$  perl -E 'my $count = 5; say $count while decr($count); sub decr {--
+$_[0];}'
4
3
2
1

[download]
Note that, in this case, $_[0] is an alias for $count, so that decrementing $_[0] does decrement $count. Not necessarily a best practice in general, but OK IMHO for a one-liner.

Or a variation of the same not using aliases: 

$  perl -E 'my $count = 5; say $count while $count = decr($count); sub
+ decr {return -1 + shift;}'
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3
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[download]

In reply to Re: User defined function in while loop by Laurent_R
in thread User defined function in while loop by kgherman

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