No need for parentheses; nor to escape the space; nor for the 'g' modifier - '.*' will gobble everything up, thus there can be only one match. So: 

perl -p -i -e "s/^macros.*/macros regex/" monks.txt

# or

perl -p -i -e "s/^macros\K.*/ regex/" monks.txt

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In reply to Re^2: Perl One Liner Regular Expressions by hbm
in thread Perl One Liner Regular Expressions by stallion

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