What is the meaning of 忽爾間 in the song "說散就散"?
I think it is more common to say 忽然间 than 忽尔间，but it is a song!

忽然间他看见了我,就往回走, 伸出手来.
Suddenly he saw me, and walked towards me, holding out his hand.

因為成長, 我們忽爾間說散就散
Because (we had) grown, changed, we suddenly said break up then broke up.

There is also 忽而 。。。 忽而，but that is a different kettle of fish!