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$re = qr[\d+\D+Y\D(\d|$)];;

...

print '\\files\builds\data\M9998SBQCACSXD30401S' =~ $re ? 'yes' : 'no'
+;;
no

$re = qr[
    \d+    # after the first (and any subsequent digits )
...
    \D     # followed by only one more non-digit
    (\d|$) # before another digit or end of string
]x;