Due to a feature of Perl called Autovivification, you only need this:

use strict;
use warnings;

while(<FGLCS>){
  chomp;
  my ($scfd_name_frgt,$stt_pst_frgt,$stp_pst_frgt,$lgh_frgt)=split /\t
+/,$_;
  $scfd_name_frgt=reverse $scfd_name_frgt;
  push @{$frgtlocs{$scfd_name_frgt}},($stt_pst_frgt,$stp_pst_frgt,$lgh
+_frgt);
}
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If @{$frgtlocs{$scfd_name_frgt}} does not exist, Perl just creates an array and you are done.

UPDATE: Also looking at these two lines:

@{$scfd_name_frgt}=($stt_pst_frgt,$stp_pst_frgt,$lgh_frgt);
$frgtlocs{$scfd_name_frgt}=\@{$scfd_name_frgt};
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They can be abbreviated using square brackets to create a reference to an array:

$frgtlocs{$scfd_name_frgt}=[$stt_pst_frgt,$stp_pst_frgt,$lgh_frgt];
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There is no need to create a named array first and then store a reference to it (if that is what you want).