Indeed it matches a sequence of one or more charachters in the range [a-g] next to the newline. Precisely "g\n". In case of doubt although there's a case against $& ;-) you may try printing that to see what matched. Or else put a pair of capturing parens and print $1 which amounts to fundamentally the same thing. Or even use the return value of the match:

$ perl -pe '($_)=/[a-g]+\n/g'
abcdef g
g
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