Looks like that 0 is a typo.
Suppose precedence makes parens unneccesary too but hooray for parens!

perl -MO=Deparse -e 
 'next LINE while (($line =~ /abcd/o) .. ($line =~ /def/0));'
Number found where operator expected at -e line 1, 
 near "/def/0" (Missing operator before 0?)
syntax error at -e line 1, near "/def/0" -e had compilation errors.
;

perl -MO=Deparse -e 
 'next LINE while (($line =~ /abcd/o) .. ($line =~ /def/o));'
-e syntax OK  
next LINE while $line =~ /abcd/o .. $line =~ /def/o;
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