my @array = ( 1, 5, 7, 9 );

sub minindex {
  my( $aref, $idx_min ) = ( shift, 0 );
  $aref->[$idx_min] < $aref->[$_] or $idx_min = $_ for 1 .. $#{$aref};
  return $idx_min;
}

print minindex(\@array), "\n";