As jZed said, had you tried, you would have found out yourself.

use Data::Dumper;

use strict;
use warnings;

my $t = "abc";
my $b = "x";
my $l = "xyzabcdef";

$l =~ s/$t/$b/g;

print $l;
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This prints:

xyzxdef
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Make it simple, obviously it works.