you need to escape the '/' in <\/a>, since it would improperly terminate the regex otherwise.

or use something other than a forward slash as your regex delimiter, for example paired curly brackets like this

$mystring =~
   s{(\d{3,5})}
   {<b><a href='lookup.cgi?id=$1</a>}g;
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You often see matching against *nix paths using the default forward slash delimiter (and omitting the m as it is not required with / ... /) when using a different delimiter would be more readable.

if( $path =~ /\/path\/to\/file/ ) { ... }
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is, I feel, much harder to read than

if( $path =~ m{/path/to/file} ) { ... }
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Cheers,

JohnGG