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    $var = "foo" ;
    $var .= "bar" ;             # 1
...
    $var = "foo$var" ;          # 2

    substr($var,0,0) = "foo" ;  # 3

#!/usr/bin/perl -w

...
    Concatenate => q/$var = "String"; $var = "foo" . $var;/,
    Substr      => q/$var = "String"; substr($var, 0, 0) = 'foo'/
});

Benchmark: timing 1500000 iterations of Concatenate, Interpolate, Subs
+tr...
Concatenate:  1 wallclock secs ( 1.56 usr +  0.00 sys =  1.56 CPU) @ 9
+60000.00/s (n=1500000)
Interpolate:  1 wallclock secs ( 1.59 usr +  0.00 sys =  1.59 CPU) @ 9
+45812.81/s (n=1500000)
    Substr:  2 wallclock secs ( 2.07 usr +  0.01 sys =  2.08 CPU) @ 72
+1804.51/s (n=1500000)