Well, if your "$buf" contains the entire content of some file (because you have read the file using "slurp" input mode), of course grep is going to return the whole thing.

If you want the specific line that contains the given value, either treat the file data as an array of lines, or else change how you are looking for the target string:

# (no need to change how $buff is assigned)

# option 1:

my ( $result ) = grep( /28/, split( /\n/, $buff ));
my $target = ( split( / /, $result )[1];  # get 2nd token from matched
+ line

# option 2:

my ( $target ) = ( $buff =~ /(\S+)\s+28\n/ );
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In the latter approach, if " 28\n" occurs more than once in the file, only the first occurrence is captured to $target.