Here is an annotated version of your code to try and demonstrate the issue:

#!/usr/bin/perl 
use strict;                     # Great start!

my @sample=qw(a,b,c);           # Declare array variable
&trial(\@sample);               # Pass array as a reference to sub
sub trial
{
   @sample=@_;                  # Clobber the contents of your origina
+l array
                                # setting the original array to the co
+ntents
                                # of the special array (@_), which jus
+t has
                                # a reference
   print join("\t",@$sample);   # Try to dereference a scalar variable
+ named
                                # 'sample', which you never declared.
}
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use strict and warnings... warnings would also show another issue with your use of qw. Here is an improved version:

#!/usr/bin/perl 
use strict;
use warnings;

my @sample = qw(a b c);
trial(\@sample);
sub trial {
   my ($aref) = @_;
   print join "\t", @{ $aref };
}
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See also References quick reference