Ignoring the symtax error, that won't work, its an infinite loop, you need 

while( $pairs =~ m{(\S+) \s/ (\S+)}xg ) {
    my( $ip, $mask ) = ( $1, $2 );
}

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See Re: Surprise: scalar(($x, $y) = split)

In reply to Re^2: Match zero times in regex by Anonymous Monk
in thread Match zero times in regex by SomeNetworkGuy

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