As others have pointed out, you are are possibly overcomplicating this by trying to use strftime()
here. You also have a misunderstanding where you are trying to call a method on a variable that doesn't contain an object.
I think that perhaps you were trying to write something like this.
#!/usr/bin/perl
use strict;
use warnings;
use 5.010;
use Time::Piece;
# The original date as a string
my $date_str = '2013-10-31 13:55';
# Parse the date string and create a Time::Piece object
my $date = Time::Piece->strptime($date_str, '%Y-%m-%d %H:%M');
# Create a new (improved!) date string using strftime()
my $new_date = $date->strftime('%Y-%m-%d %H:%M:%S');
say $new_date;
Time::Piece has been a standard Perl library for several years.