cat << EOF | perl -pe 's{^\s*\n$}{}; s/ +(?=(([^"]+"){2})*[^"]*$)//g'
This is an example text with some spaces.
This should be 2nd line.
However the spaces between "quotes should not change".
last line.
EOF
Y2F0IDw8IEVPRiB8IHBlcmwgLXBlICdze15ccypcbiR9e307IHMvICsoPz0oKFteIl0rIil7Mn0pKlteIl0qJCkvL2cnClRoaXMgaXMgYW4gZXhhbXBsZSB0ZXh0IHdpdGggICAgc29tZSAgICAgIHNwYWNlcy4gCgoKClRoaXMgc2hvdWxkIGJlIDJuZCBsaW5lLgoKSG93ZXZlciB0aGUgc3BhY2VzIGJldHdlZW4gInF1b3RlcyAgICBzaG91bGQgbm90ICAgIGNoYW5nZSIuCgpsYXN0IGxpbmUuCkVPRg==