Basically that means some word followd by a '_' and '-'.

Well actually no it doesn't, see perlintro#More complex regular expressions

item 8. matches a single character in the given set, so it matches a single word character or dash, but only one character

if you want to match it more than once, you have to use a quantifier like item 1 or item 2 from the quantifier list; or the second Quantifiers list in perlre

So you might write

m{^Users of (.+?): \(Total of (.+?) licenses issued; Total of (.+?) li
+cense in use\)$}im
m{^Users of ([^:]*): \(Total of (.+?) licenses issued; Total of (.+?) 
+license in use\)$}im
m{^Users of ([^:]+): \(Total of (\d+) licenses issued; Total of (\d+) 
+license in use\)$}im
m{^Users of ([\w\_\-]+): \(Total of (\d+) licenses issued; Total of (\
+d+) license in use\)$}im
m{^Users of ([\w\_\-]+): \(Total of (\d+) licenses issued; Total of (\
+d+)}im
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Modern Perl ch 6 is also an introduction to regex