The ++ is after variable, so it increments after you test it; however, the print is a separate statement. If you used Compound Statements instead of Statement Modifiers, your code would better reflect the actual flow and would look like:

$i=0;
if ($i++) {
    print 'first=' , $i, ', ';
}
if ($i++) {
    print 'middle=' , $i, ', ';
}
if ($i++) {
    print 'last=' , $i, ', ';
}
if ($i++) {
    print 'beyond=' , $i, ', ';
}
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For what you want, you probably mean:

$i=0;
if ($i++) {
    print 'first=' , $i - 1, ', ';
}
if ($i++) {
    print 'middle=' , $i - 1, ', ';
}
if ($i++) {
    print 'last=' , $i - 1, ', ';
}
if ($i++) {
    print 'beyond=' , $i - 1, ', ';
}
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which is admittedly a little awkward. More natural would be to post-increment after your print, except you clearly intend the incrementation to happen independently of whether the print happens. Perhaps

$i=0;
print 'first=' , $i, ', ' if $i;
$i++;
print 'middle=', $i, ', ' if $i;
$i++;
print 'last='  , $i, ', ' if $i;
$i++;
print 'beyond=', $i if $i;
$i++;
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or

my $i=0;
outputter('first', $i++);
outputter('middle', $i++);
outputter('last', $i++);
outputter('beyond', $i++);

sub outputter {
    my ($txt, $i) = @_;
    print "$txt=$i, " if $i;
}
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