main.pl:

#/usr/bin/perl

print "Entry point\n";
log('started');

sub log {
# do the usual to open the logile

# now print it
print LOG @_;

# close it
}

##</code><code>##

package.pl:

package mutex;

sub update {
# do something and log it
&main'log("can't do something");
}

1;