use strict;
use warnings;
my $value = '103a';
my $IsNumber = '/^\d+$/';

print "Number\n" if (eval ('$value =~ '."$IsNumber"));
print "No number\n" if (eval ('$value !~ '."$IsNumber"));

__END__
Out:
No number

##</code><code>##

my $IsNumber = '/^\d+$/';

##</code><code>##

'$value =~ '."$IsNumber"