The following will extract the leading digits from $fields[0] and place them in $maid:

my ($maid) = $fields[0] =~ /^(\d*)/;
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In scalar context, the match op would return a value indicating whether the match was successful or not. That's useless to you since the pattern will always match. In list context, the match operator returns a list of a all the captures.

The parens around $maid are necessary to select the list assignment operator which places the match operator in list context.