Just want to point out that you can also using symbolic substitution, which was how I interpreted the OP's "substituting certain keywords with variables".   Although I made a recent argument in favor of always being in the habit of using strict, I can see why this might be a good situation for using "no strict 'refs'":

#!/usr/bin/perl -w
                                                                      
+          
use warnings;
use strict;
no strict 'refs';
                                                                      
+          
our $a = 10;
our $b = 20;
our $c = 33;
our $d = 123;
                                                                      
+          
my @lines = qw( <:a:0:> <:a:3:> <:b:0:> <:b:4:> <:c:5:> <:d:6:> );
                                                                      
+          
foreach my $line (@lines) {
    if ($line =~ s/<:(\w+):(\d+):>/$$1*($2||1)/e) {
        printf "Result = %d\n", $line;
    }
}
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Output is:

    Result = 10
    Result = 30
    Result = 20
    Result = 80
    Result = 165
    Result = 738