The general form is:

  <statement> <modifier>
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and it is roughly equivalent to:

  <modifier>
  {
    <statement>
  }
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The modifiers can take the form of a conditional, or a looping construct:

  <statement> if $a > 1;
  <statement> unless $true;
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  <statement> for @a;
  <statement> for 1..10;
  <statement> for keys %hash;
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The rule is that there can only be one modifier, so while there can be a complex logical condition, there cannot be two distinct conditions, or a mix of condition and loop. So the following are not valid:

  <statement> <modifier> <modifier>

  <statement> for @a unless $true;
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