Regexp::Common::net also provides regexes for parsing IPv4 addresses.

Assigns everything up to the first comma to $first:

my ($first) = $str =~ /(.*?),/;
[download]

Not as efficient, but possibly more readable:

my ($first) = split(/,\s*/, $str);
[download]

Update: D'oh! Fixed bug mentioned in reply.

Assigns everything up to the first comma to $first

$ perl -wle 'my $str = "a, b,"; my ($first) = $str =~ /(.*),/; print $
+first'
a, b
[download]

Up to the last comma, actually. If you want "up to the first comma" either use ([^,]*), or (.*?),

I think the split() solution is more efficient than the regex solution.

Tested with Perl 5.8.8 (ActiveState on WinXP)

#!/usr/bin/perl
use strict;
use warnings;
use Benchmark qw( cmpthese );

my $str = ( ('X'x15) . ',' ) x 3;

cmpthese(
    -1,
    {
        # using regex
        's///' => sub {
            my ( $first ) = $str =~ m/(.*?),/;
        },
        # using split without limit
        'split_unl' => sub {
            my ( $first ) = split( m/,/, $str );
        },
        # using split with limit
        'split_lim' => sub {
            my ( $first ) = split( m/,/, $str, 2 );
        },
        # using split with limit and slice for scalar assignment
        'split_lim_sca' => sub {
            my $first = ( split( m/,/, $str, 2 ) )[0];
        },
    }
)
[download]

my result:

                  Rate          s///     split_lim     split_unl split
+_lim_sca
s///          577066/s            --          -13%          -13%      
+    -15%
split_lim     663032/s           15%            --           -0%      
+     -2%
split_unl     663629/s           15%            0%            --      
+     -2%
split_lim_sca 679348/s           18%            2%            2%      
+      --
[download]

Or did I miss something?

and here's my result with a perl 5.10 (cygwin on same WinXP)

                  Rate          s///     split_lim     split_unl split
+_lim_sca
s///          288797/s            --          -12%          -12%      
+    -15%
split_lim     326805/s           13%            --           -1%      
+     -4%
split_unl     330050/s           14%            1%            --      
+     -3%
split_lim_sca 339541/s           18%            4%            3%      
+      --
[download]

I would use

my $first = ( split /,\s*/, $line, 2 )[0]
[download]

to extract the first element.

I would do the test if $first is an ip address afterwards.

What moritz said. This would be a start, which you might need to tweak based on any unmentioned requirements:

use Regexp::Common qw(net);

while (<>) {
    my ($keeper) = $_ =~ /
        ($RE{net}{IPv4})    # Capture the IP
        ,                   # comma
        /x;
    printf "%s\n", $keeper if $keeper;
}
[download]

$strout = (split(',',$line))[0];
[download]

Why has no one suggested that OP have a look at How do I post a question effectively? yet?

Message to OP: We will never usually not write your code for you. Please show some effort first.