- ^ is match at beginning ..
- \d is looking for a digit character
- + is find \d 1 or more times
- (...) Groups subexpressions for capturing to $1, $2,$3 ...

In this case, $scrip will return the the complete sub expression match (which is inside the brackets : (\d+\.\d+\.\d+\.\d+) ):

my ($scrip) = $ARGV[0] =~ /^(\d+\.\d+\.\d+\.\d+)/;

print "\$scrip returned : $scrip\n";
print "match 1 : $1\nmatch 2 : $2\nmatch 3 : $3\nmatch 4 : $4\n";
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INPUT
C:\Perl\bin>perl bleh.pl 1.0.3.3_1.3.45.44

OUTPUT
$scrip returned : 1.0.3.3
match 1 : 1.0.3.3
match 2 :
match 3 :
match 4 :

If "\d+\.\d+\.\d+\.\d+" wasn't inside the brackets, $scrip would return 1, if it matched the reg-ex:

my ($scrip) = $ARGV[0] =~ /\d+\.\d+\.\d+\.\d+/;

if($scrip) {
    print "matched $ARGV[0] !\n";
}else{
    print "did not match $ARGV[0] !\n";
}
print "\$scrip = $scrip\n";
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OUTPUT EXAMPLES:

C:\Perl\bin>perl bleh.pl 13.3.3.3_
matched 13.3.3.3_ !
$scrip = 1

C:\Perl\bin>perl bleh.pl 13.3.
did not match 13.3. !
$scrip =