use B::Deparse;

sub something {
    my ( $self, $param ) = @_;
    return _stuff($param);
}

my $subref = \&something;
my $code = B::Deparse->new->coderef2text($subref);
print "Before editing &something:\n",$code;
$code =~ s/\{/\{\nlog_msg('starting sub');\n/;
*something = eval "sub $code";

$code = B::Deparse->new->coderef2text(\&subref);
print "\nAfter editing &something:\n",$code;

##</code><code>##

Before editing &something:
{
    use warnings;
    use strict 'refs';
    my($self, $param) = @_;
    return _stuff($param);
}
After editing &something:
{
    use warnings;
    use strict 'refs';
    log_msg('starting sub');
    my($self, $param) = @_;
    return _stuff($param);
}