use strict;

my $x = "12345678901234";
# This evalutes the regex and prints the result
print $x =~ /^(\d{10})/;
print "\n";

$x = "12345678901234";
# This evaluates the regex but the result is not assigned anywhere. So
+ you get back $x
$x =~ /^(\d{10})/;
print "$x\n";

$x = "12345678901234";
# This evaluates the regex and assigns the result to $y
my ($y) = $x =~ /^(\d{10})/;
print "$y\n";

$x = "12345678901234";
# This perform a substitution, returning the result in $x 
$x =~ s/^(\d{10})\d*$/$1/;
print "$x\n";
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Update:
Did not see post by Eimi Metamorphoumai (below my depth setting)
This is somewhat of a repeat.