Your Mother is spot on, just match instead of trying to replace.

Regarding your core question, you have multiple problems

• you don't anchor the pattern between literal parens \( ... \)
• your trailing quantifier is non-greedy .*? and is satisfied after matching 0 characters, i.e. the rest is not replaced
• this could also have been fixed by additionally anchoring on ^start and end$ to really replace the whole string

Anyway here a demo that works

C:\tmp>perl
my $ip = "csteppfw pts/0        2022-06-27 15:53 (192.168.5.100)";
$ip =~ s/.*\((\d+\.\d+\.\d+\.\d+)\).*/\1/;
print $ip;
__END__
192.168.5.100
C:\tmp>
[download]

But still, simply matching is better.

C:\tmp>perl
my $ip = "csteppfw pts/0        2022-06-27 15:53 (192.168.5.100)";
print $1 if $ip =~ /\((\d+\.\d+\.\d+\.\d+)\)/;
__END__
192.168.5.100
C:\tmp>
[download]