Please help me understand the below reg expressions:

    /^\s*$/
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means check for all spaces in the entry. Check for beginning spaces and spaces in the middle and ending with a space??

It means: match the beginning of the string, followed by zero or more whitespace characters, followed by the end of the string. Essentially, it matches what we often think of as a "blank" line.

    /^\w{1,}@/
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Check for beginning with one word only and a '@' ??

Match beginning of string, followed by 1 or more word characters (those being of the class: [a-zA-Z0-9_]) followed by the '@' character.

To really understand regular expressions better, please see the following documents: perlrequick, perlretut, perlre. Also, Jeffrey Friedl's "Mastering Regular Expressions" is an excellent resource.