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while ( ( my $index = index substr( $_, pos() ), $find),
          $index != -1 )
{
    substr $_, $index, length $find, $replace;
    pos() = $index + length $find;
}

while ( ( my $index = index $_, $find),
          $index != -1 )
{
    substr $_, $index, length $find, $replace;
}