Hi,
A little breakdown of the commands:
1) $IP_eth0 = `ifconfig eth0`;
2) $IP_eth0 =~ s/.*inet addr: (.*) Bcast:/1/;
3) print "IP eth0 => " . $1 . "\n";
The first line is probably clear: fill $IP_eth0 with the output of the ifconfig eth0 command (ifconfig eth0 is not a pearl command, but a unix command).
The second line uses one of perl's regular expression ( s/regex/replacement/modifiers ) to 'cut out' the part you want.
In this case all the output stored in $IP_eth0 is checked and if the following
.*inet addr: (.*) Bcast: is found, the part between the ( and ) can be represented as
$1, so ( and ) are not part of the regular expression, they are special and can be used in the replacement part of the regular expression.
Take a look here
perlrequick and here
perlretut for a more detailed explanation.
The third line prints $1, which is what is set after the substitute command, the part between the ( and ), in the previous line.
Hope this clears things up.