(simplified to some extent)
the two /'s specify the matching operator (also seen as m//). this match operator allows you to search for certain regular expressions in a string. in this case, you are searching for 'is' which gets captured by the parentheses. this means that 'is' gets stored in the \1 variable. so you are searching for 'is' then a single space, then another 'is'. so where are you searching? well, the matching operator defaults to the $_ special variable (doesn't everything?). so you are looking in $_ for 'is is'. you can also specify a string to search in using the binding operator =~. so this expression could also be written as:

$_ =~ m/is is/  # or
$_ =~ m/(is) \1/
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the parentheses around the statement simply means that this is a test (for an if or a while or something).

you should really check out the perlre manpage or 'Learning Perl' by O'Reilly or something for a much more detailed explanation of regular expressions.

jeff