In Perl, how can I tell if a string is a number, but without using modules?

"The first word is identifier, which will not be considered. The rest part of the string would be number or alphabet only. "

#!/usr/bin/perl -w

use strict;
use warnings;
#import Scalar::Util qw(look_like_number);

my @log=("a1 9 2 3 1","gl act car","zo4 4 7", "abl off key dog","a8 act zoo");
print "original log:\n";
$"=";\t";   #control array's delimiter
print "@log\n";

my @values;
my @letter_log;
my @letter_idf;
my @dig_log;
my $s;
my $i;
my $s_r;

$i=0;
foreach(@log)
{
  @values=split(/\s+/,$_);chomp(@values);
  @_= m/$values[0]\s/;
print "25 \$'=$';\n";
  $s=$';
  $s_r=substr($s,0,1);
  if($s_r =~ /\D/)  
  {  print"26.0: $'\ts=$s\ts_r=$s_r\n";    
     $letter_idf[$i]=$values[0];
     $letter_log[$i]=$s;
     print "26:  $letter_idf[$i]\t$letter_log[$i]\n";$i++;
  }
  else{$dig_log[$i]=$_;}
}

@_=sort { $letter_log[$a] cmp $letter_log[$b] } 0..$#letter_log;
chomp(@_);

print"35:  srt letter  log: @letter_log\n";
#foreach(@letter_log){print "$_;\t";}

print"38: nsrt letter log: ";
for($i=0;$i<=$#letter_log;$i++){print"$letter_idf[$_[$i]] "."$letter_log[$_[$i]];\t";}
print"\n";
print"41: digital log:";
foreach(@dig_log){print "$_;\t";}

print "\n";