my $string = qq~'tom's is silly','some','peter said it's okay'~;

$string =~ s/(?!,)'s/\\'s/g;

print "$string\n";

#output: 'tom\'s silly',\'some','peter said it\'s okay'

#desired output: 'tom\'s is silly','some','peter said it\'s okay'

# In the desired output, the single quote before "some" is not escaped