The clearest way of describing the difference I can think of is to fully expand the syntax

print ++$i;
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is equivalent to

$i = $i +1; 

print $i;
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whereas

print $i++;
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is equivalent to

print $i; 

$i = $i + 1;
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which I think emphasises the order in which things occur. The difference only becomes material when you use the pre- or post-increment notation as part of another statement. Ie. ++$i; and $i++; as stand-alone statements have identical effect.

The difference only becomes material when the syntax is used as a part of another statement.

Which can be exemplified by

my @a = (1..10);

my $i = 0; 
print $a[$i++] while $i < @a;

12345678910

$i = 0; 

print $a[++$i] while $i < @a;

2345678910
Use of uninitialized value in print at (eval 8) line 1, <> line 7.
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