Yes, perlre is the right direction :-) You can use character class where you can group some characters based on certain specification, and in this case you can make arbtrary spec. Perl provides some symbols for predefined character class, such as \w for alphabet and numeric and underscore ("_"), \d for digits only, and \s for all kind of recognized spaces.

From your description I think you need the \w character class,

$ perl -le 'print "hello world" =~ /^\w+$/ ? "OK" : "BAD"'
BAD ---> contain spaces
$ perl -le 'print "helloworld" =~ /^\w+$/ ? "OK" : "BAD"'
OK
$ perl -le 'print "hello_world" =~ /^\w+$/ ? "OK" : "BAD"'
OK ---> underscore is considered alphanumeric character.
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If you want to exclude the underscore as well then you have to say the character class explicitly,

$ perl -le 'print "hello_world" =~ /^[a-zA-Z0-9]+$/ ? "OK" : "BAD"'
BAD ---> underscore is not in the class
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Since version 5.6.0, Perl supports [:class:] style of character class, and the equivalet for above is [:alnum:].

$ perl -le 'print "hello world" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
BAD ---> space is not alnum
$ perl -le 'print "hello_world" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
BAD ---> _ is not alnum
$ perl -le 'print "HelloWorld1" =~ /^[[:alnum:]]+$/ ? "OK" : "BAD"'
OK --> nothing there but letters and digits
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HTH,