clear formatting might help you see the function was embedded in a double-quoted string, and was not interpolated as a function. try (untested)

$page .= p( 
  { -style => "font-family: verdana; font-size: 10pt;" }, 
  "Shipping and tax may be added to your order.\n"
  . "For estimated shipping and tax, see " 
  . a( 
    { -href => "../shiptax.htm" }, 
    "Shipping and Tax Estimate"
  )
);
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~Particle *accelerates*