The error, translated to newbie terms, is that you didn't define $inp
You define your variables using mymost of the time.

Adding myreveals a slightly less obvious error -- you neglected to end your first printstatement with semi-colon. :-)

Because you wisely did use strict;, it gave the following errors:

D:\PerlMonks>digits1.pl
syntax error at D:\PerlMonks\digits1.pl line 5, near "chomp"
Global symbol "$inp" requires explicit package name at D:\PerlMonks\di
+gits1.pl line 5.
Global symbol "$inp" requires explicit package name at D:\PerlMonks\di
+gits1.pl line 6.
Global symbol "$inp" requires explicit package name at D:\PerlMonks\di
+gits1.pl line 7.
Execution of D:\PerlMonks\digits1.pl aborted due to compilation errors
+.

D:\PerlMonks>
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Fixed:

use strict;
use warnings;

print 'Please enter your card number';
chomp(my $inp = <>);
$inp =~ s/(\d{4})(\d{4})(\d{4})(\d{4})/$1 $2 $3 $4/;
print $inp,"\n";
[download]