"is not working" is not really an adequate problem description. Please see I know what I mean. Why don't you?

But take a look at your regular expression: \d{1,3} vs. \d{1-3} - why the difference? One is right, the other is wrong. See perlrequick and perlretut.

Note that even if you fix that, the regular expression will still match invalid IP's like 999.999.999.999. Instead of reinventing the wheel, I'd recommend Regexp::Common::net:

use warnings;
use strict;
use Regexp::Common qw/net/;

my $output = "ae4.5784     up    down inet     182.75.123.121/30";

if ( $output =~ /\b$RE{net}{IPv4}\b/ ) {
    print "There IS an IPAddress in the output string\n";
}
else {
    print "There is NO IPAddress in the output string\n";
}
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Update: Added the word boundary \b to the above regex, because otherwise it would have matched something like 1111.1.1.1 (because 111.1.1.1 is a valid IP) or 256.255.255.256 (because 56.255.255.25 matches).